Add AsRefSource to PyNativeType.

[davidhewitt]

This adds an associated type AsRefSource to PyNativeType.

This allows type inference in the reverse direction &T -> Py<T>, which is particularly useful for &PyCell<T> -> Py2<T> . It also helps for forwarding our gil-ref methods to the new traits, e.g. by solving the inference problem in #3652

Really this is most helpful when we're in a transition phase where users might be using Py2::borrowed_from_gil_ref themselves. So I'm also happy to decide that we don't merge this until we're ready to release Py2 as a whole, and for now we just continue to add type annotations while we're landing the groundwork internally.

[adamreichold]

I don't think we should hold off on the quality-of-life improvements for internal work as it makes this faster.

I am currently not seeing a downside besides a general unease with more coupling within our already complex trait landscape. Is there anything in particular that one should be aware of?

[davidhewitt]

Thanks; yes this makes migration work nicer for sure. I think the only downside is that manual implementors of PyNativeType will need to add this associated type for very little immediate benefit, depending on whether the Py2 stuff makes it into 0.21 (still unsure how fast we're going to manage to go).

In the longer term, I think we can throw away the whole PyNativeType trait when we get rid of the pool and gil refs. So we have to ride over a peak of complexity in the short term and can then simplify again.

[davidhewitt]

(it should be noted that I don't expect there to be downstream manual implementors of PyNativeType, but you never know...)