dart/ffi: automatic release of pointers

[derolf]

Right how, pointers returned from FFI functions require manual reference counting on the dart level.

Provide a way to automatically clean up native references once they are not referenced anymore.

[dcharkes]

That can potentially be achieved in two ways:

• pool allocation: clean up when end of scope (like try-with-resources in Java or using in C#)
• finalizers: clean up when garbage collector collects pointer object

Pool allocation is tracked in dart-archive/ffi#30. We would like to add the pool to package:ffi soon, please leave your feedback on that issue.

Finalizers is tracked in #35770. This will take longer to implement.

I'll close this issue as a duplicate of either one of the above two. Feel free to reopen if you have questions related which are not pool or finalizer related.