Resource tracking 
=================

resource:notify_when_destroyed(Pid, Message) -> NifRef.

notify_when_destroyed creates a nif resource that can be used as a reference
to any kind of external data. When resource is destroyed, 
beacuse it is not referenced any where, the destructor callback will
send the message to Pid.

For example, it could be used for wrapping
file handles. The file is then closed automatically when there are no one
referencing the file, thereby garbage collect files.