Add IntoIterator for &Bound types

[LilyFoote]

This allows using common types like &Bound<'py, PyList> in a for loop without explicitly calling iter().

[Icxolu]

This looks useful to me, maybe it would make sense to also do this for the other iterables (PyTuple, Py(Frozen)Set and PyDict come to mind)?

[davidhewitt]

Thanks, I think these do make sense, I since remembered that I hit this a fair bit in pydantic-core when testing out the new Bound API there.

Some history on why these didn't yet exist is in #3695 (comment) and #3694 (comment) - basically I think @adamreichold and I had held back on adding these previously because I'd forgotten the use case at the time :)

I think the only question I have left remaining is what to do about &Bound<'_, PyIterator>? It might be nice to have IntoIterator for that one too, but I'm less sure.

[davidhewitt]

I think let's merge this as-is. I think the only possible design question is whether we would want to have extra iterator types that contain &Bound instead of Bound.

The only advantage of the extra types would be to be able to avoid one reference count operation, and iteration necessarily is N reference counting ops anyway. Due to this I think it's not worth bothering with the extra complexity of additional types.