Add as_borrowed conversion from gil-refs to Bound<T>

[davidhewitt]

Update: we changed to add .as_borrowed() instead of .as_bound() as it gave additional flexibility.

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This is the promised follow-up to #3686 which adds a .as_bound() method to the GIL refs API types to convert into the new Bound<T> smart pointer.

I've switched all internal code over to use this API to give a feel of how it reads; I think it's much nicer to type and read self.as_bound() over Bound::borrowed_from_gil_ref(&self). Hopefully this helps users to migrate.

I think the next PR I plan to open will be a beginning of the migration guide entry; I'd just like this conversion API agreed upon first as this will form a key part of migration instructions.

[davidhewitt]

Well, I added commits for all the suggestions here, but this PR has gotten a bit bulky as a result.

I've split some pieces and related changes into #3694, #3695, #3697 and #3698.

Let's review those separately and then I'll come back to this one to rebase it and we can review again.

[davidhewitt]

Given that this PR is one of those PRs which conflicts with a lot of the remaining PRs which I'd like to open wrt PyXMethods trait implementations, I've come back here and dropped all the commits which are addressed in other PRs.

That means that this should now be reviewable / mergeable again and the only feedback not addressed here is the PyIterator::from_object2 change which is open to merge separately in #3702.

If this one does merge I'll rebase all the other PRs which conflict with it 🧑‍💻