A safe API for upcasting

[davidhewitt]

#770 initially implemented AsRef<Base> for PyRef<T>.

We had to remove that in #783 for native base types due to a safety issue in the implentation.

It would be ideal if AsRef could be implemented for PyRef<T> for both T and all bases of T - e.g. AsRef<T>, AsRef<T::Base>, AsRef<T::Base::Base> etc.