Add finalizers

[alexbatalov]

This PR adds finalizers to native objects. Note that I'm using sp_close as a SerialPort finalizer which is much more important than sp_free_port since it allows to re-open serial port after Flutter's Hot Restart.

While it's perfectly fine to have more than one native finalizer for the same object, Dart SDK does not make guarantees about their execution order. So if you try to add sp_free_port finalizer as well, it might be called before sp_close finalizer (depending on the environment or Dart SDK version). Ideally underlying libserialport will have something like sp_dispose with deterministic order (close and free).

Closes #68