feat: Packages in hugr-model and envelope support.

[zrho]

This PR adds packages to hugr-model with support for envelopes.

• Packages for both the AST and table representation of hugr-model.
• Packages in the Python model classes.
• Reading and writing model envelopes in Rust.
• Writing model envelopes in Python.

[hugrbot]

This PR contains breaking changes to the public Rust API.
Please deprecate the old API instead (if possible), or mark the PR with a ! to indicate a breaking change.

cargo-semver-checks summary

--- failure enum_missing: pub enum removed or renamed ---

Description:
A publicly-visible enum cannot be imported by its prior path. A `pub use` may have been removed, or the enum itself may have been renamed or removed entirely.
      ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
     impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.40.0/src/lints/enum_missing.ron

Failed in:
enum hugr_core::hugr::rewrite::BoundaryPort, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/hugr/rewrite/port_types.rs:14

--- failure enum_no_repr_variant_discriminant_changed: enum variant had its discriminant change value ---

Description:
The enum's variant had its discriminant value change. This breaks downstream code that used its value via a numeric cast like `as isize`.
      ref: https://doc.rust-lang.org/reference/items/enumerations.html#assigning-discriminant-values
     impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.40.0/src/lints/enum_no_repr_variant_discriminant_changed.ron

Failed in:
variant EnvelopeError::SerdeError 6 -> 5 in /home/runner/work/hugr/hugr/PR_BRANCH/hugr-core/src/envelope.rs:163
variant EnvelopeError::IO 7 -> 6 in /home/runner/work/hugr/hugr/PR_BRANCH/hugr-core/src/envelope.rs:168
variant EnvelopeError::Package 8 -> 7 in /home/runner/work/hugr/hugr/PR_BRANCH/hugr-core/src/envelope.rs:173
variant EnvelopeError::PackageEncoding 9 -> 8 in /home/runner/work/hugr/hugr/PR_BRANCH/hugr-core/src/envelope.rs:178
variant EnvelopeError::ModelImport 10 -> 9 in /home/runner/work/hugr/hugr/PR_BRANCH/hugr-core/src/envelope.rs:184
variant EnvelopeError::ModelRead 11 -> 10 in /home/runner/work/hugr/hugr/PR_BRANCH/hugr-core/src/envelope.rs:190
variant EnvelopeError::ModelWrite 12 -> 11 in /home/runner/work/hugr/hugr/PR_BRANCH/hugr-core/src/envelope.rs:196

--- failure enum_variant_missing: pub enum variant removed or renamed ---

Description:
A publicly-visible enum has at least one variant that is no longer available under its prior name. It may have been renamed or removed entirely.
      ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
     impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.40.0/src/lints/enum_variant_missing.ron

Failed in:
variant EnvelopeError::MultipleHugrs, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/envelope.rs:167

--- failure function_missing: pub fn removed or renamed ---

Description:
A publicly-visible function cannot be imported by its prior path. A `pub use` may have been removed, or the function itself may have been renamed or removed entirely.
      ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
     impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.40.0/src/lints/function_missing.ron

Failed in:
function hugr_core::import::import_hugr, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/import.rs:83
function hugr_core::export::export_hugr, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/export.rs:31

--- failure inherent_method_missing: pub method removed or renamed ---

Description:
A publicly-visible method or associated fn is no longer available under its prior name. It may have been renamed or removed entirely.
      ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
     impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.40.0/src/lints/inherent_method_missing.ron

Failed in:
StaticArrayValue::get_contents, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/std_extensions/collections/static_array.rs:73
SimpleReplacement::into_replacement, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/hugr/rewrite/simple_replace.rs:64
SimpleReplacement::is_valid_rewrite, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/hugr/rewrite/simple_replace.rs:75
SimpleReplacement::get_replacement_io, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/hugr/rewrite/simple_replace.rs:97
SimpleReplacement::incoming_boundary, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/hugr/rewrite/simple_replace.rs:111
SimpleReplacement::outgoing_boundary, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/hugr/rewrite/simple_replace.rs:151
SimpleReplacement::host_to_host_boundary, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/hugr/rewrite/simple_replace.rs:189
SimpleReplacement::map_host_output, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/hugr/rewrite/simple_replace.rs:223
SimpleReplacement::map_replacement_input, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/hugr/rewrite/simple_replace.rs:238
SimpleReplacement::all_boundary_edges, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/hugr/rewrite/simple_replace.rs:253
SimpleReplacement::into_replacement, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/hugr/rewrite/simple_replace.rs:64
SimpleReplacement::is_valid_rewrite, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/hugr/rewrite/simple_replace.rs:75
SimpleReplacement::get_replacement_io, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/hugr/rewrite/simple_replace.rs:97
SimpleReplacement::incoming_boundary, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/hugr/rewrite/simple_replace.rs:111
SimpleReplacement::outgoing_boundary, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/hugr/rewrite/simple_replace.rs:151
SimpleReplacement::host_to_host_boundary, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/hugr/rewrite/simple_replace.rs:189
SimpleReplacement::map_host_output, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/hugr/rewrite/simple_replace.rs:223
SimpleReplacement::map_replacement_input, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/hugr/rewrite/simple_replace.rs:238
SimpleReplacement::all_boundary_edges, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/hugr/rewrite/simple_replace.rs:253
SimpleReplacement::into_replacement, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/hugr/rewrite/simple_replace.rs:64
SimpleReplacement::is_valid_rewrite, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/hugr/rewrite/simple_replace.rs:75
SimpleReplacement::get_replacement_io, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/hugr/rewrite/simple_replace.rs:97
SimpleReplacement::incoming_boundary, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/hugr/rewrite/simple_replace.rs:111
SimpleReplacement::outgoing_boundary, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/hugr/rewrite/simple_replace.rs:151
SimpleReplacement::host_to_host_boundary, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/hugr/rewrite/simple_replace.rs:189
SimpleReplacement::map_host_output, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/hugr/rewrite/simple_replace.rs:223
SimpleReplacement::map_replacement_input, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/hugr/rewrite/simple_replace.rs:238
SimpleReplacement::all_boundary_edges, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/hugr/rewrite/simple_replace.rs:253
SimpleReplacement::into_replacement, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/hugr/rewrite/simple_replace.rs:64
SimpleReplacement::is_valid_rewrite, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/hugr/rewrite/simple_replace.rs:75
SimpleReplacement::get_replacement_io, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/hugr/rewrite/simple_replace.rs:97
SimpleReplacement::incoming_boundary, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/hugr/rewrite/simple_replace.rs:111
SimpleReplacement::outgoing_boundary, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/hugr/rewrite/simple_replace.rs:151
SimpleReplacement::host_to_host_boundary, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/hugr/rewrite/simple_replace.rs:189
SimpleReplacement::map_host_output, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/hugr/rewrite/simple_replace.rs:223
SimpleReplacement::map_replacement_input, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/hugr/rewrite/simple_replace.rs:238
SimpleReplacement::all_boundary_edges, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/hugr/rewrite/simple_replace.rs:253

--- failure struct_missing: pub struct removed or renamed ---

Description:
A publicly-visible struct cannot be imported by its prior path. A `pub use` may have been removed, or the struct itself may have been renamed or removed entirely.
      ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
     impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.40.0/src/lints/struct_missing.ron

Failed in:
struct hugr_core::hugr::rewrite::HostPort, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/hugr/rewrite/port_types.rs:23
struct hugr_core::hugr::rewrite::ReplacementPort, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/hugr/rewrite/port_types.rs:27

--- failure inherent_method_missing: pub method removed or renamed ---

Description:
A publicly-visible method or associated fn is no longer available under its prior name. It may have been renamed or removed entirely.
      ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
     impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.40.0/src/lints/inherent_method_missing.ron

Failed in:
CodegenExtsBuilder::add_static_array_extensions, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-llvm/src/extension/collections/static_array.rs:41
CodegenExtsBuilder::add_default_static_array_extensions, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-llvm/src/extension/collections/static_array.rs:47
CodegenExtsBuilder::add_static_array_extensions, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-llvm/src/extension/collections/static_array.rs:41
CodegenExtsBuilder::add_default_static_array_extensions, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-llvm/src/extension/collections/static_array.rs:47

--- failure module_missing: pub module removed or renamed ---

Description:
A publicly-visible module cannot be imported by its prior path. A `pub use` may have been removed, or the module may have been renamed, removed, or made non-public.
      ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
     impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.40.0/src/lints/module_missing.ron

Failed in:
mod hugr_llvm::extension::collections::static_array, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-llvm/src/extension/collections/static_array.rs:1

--- failure struct_missing: pub struct removed or renamed ---

Description:
A publicly-visible struct cannot be imported by its prior path. A `pub use` may have been removed, or the struct itself may have been renamed or removed entirely.
      ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
     impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.40.0/src/lints/struct_missing.ron

Failed in:
struct hugr_llvm::extension::collections::static_array::StaticArrayCodegenExtension, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-llvm/src/extension/collections/static_array.rs:36
struct hugr_llvm::extension::collections::static_array::DefaultStaticArrayCodegen, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-llvm/src/extension/collections/static_array.rs:342

--- failure trait_missing: pub trait removed or renamed ---

Description:
A publicly-visible trait cannot be imported by its prior path. A `pub use` may have been removed, or the trait itself may have been renamed or removed entirely.
      ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
     impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.40.0/src/lints/trait_missing.ron

Failed in:
trait hugr_llvm::extension::collections::static_array::StaticArrayCodegen, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-llvm/src/extension/collections/static_array.rs:148