implement iterator for Bound iterator

[davidhewitt]

Implements the ability to iterate Bound<'_, PyIterator> and &Bound<'_, PyIterator> in the same way that &'py PyIterator can already be iterated.

[davidhewitt]

Right, I've cleaned up this code and also cherry-picked the commit exposing from_bound_object from #3692

[codspeed-hq]

CodSpeed Performance Report

Merging #3695 will not alter performance

_{Comparing davidhewitt:bound-iterator (d669a94) with main (6ca63b5)}

Summary

✅ 78 untouched benchmarks

[davidhewitt]

Uff, that second commit depends on #3694 so to avoid a deadlock I've split that out into #3702. This PR is the first of the three that is mergeable 😮‍💨