port PyObject::downcast to Bound API

[Icxolu]

Part of #3684

This ports the PyObject::downcast(_unchecked) methods to the Bound API.

Edit:
I also added included Python::(checked_)cast_as here. We decided to deprecate these in favor of the PyObject methods.

[codspeed-hq]

CodSpeed Performance Report

Merging #3856 will degrade performances by 15.63%

_{Comparing Icxolu:pyobject-downcast (e507287) with main (5f42c02)}

Summary

⚡ 1 improvements
❌ 3 regressions
✅ 75 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	main	Icxolu:pyobject-downcast	Change
❌	mapping_from_dict	300 ns	355.6 ns	-15.63%
⚡	f64_from_pyobject	516.7 ns	461.1 ns	+12.05%
❌	sequence_from_list	300 ns	355.6 ns	-15.63%
❌	sequence_from_tuple	232.8 ns	267.2 ns	-12.89%

[davidhewitt]

Thanks for picking this one up! Just one piece which I think we can tweak further.

[davidhewitt]

Good idea, I also had some similar pieces in #3706 and maybe I can build on top of this API 👍

[davidhewitt]

downcast APIs, look good! I wonder about a different strategy for the cast_as APIs? (See comment.)

[davidhewitt]

Given the semantics of the "replacement" is slightly different, maybe something like the following wording is more helpful to users:

Suggested change

	note = "`Python::checked_cast_as` will be replaced by `PyObject::downcast_bound` in a future PyO3 version"
	note = "part of the deprecated GIL Ref API, to migrate use `obj.downcast_bound::<T>(py)` instead of `py.cast_as::<T>(obj)`"

What do you think of this (and similar below)?

[Icxolu]

Sounds good

[davidhewitt]

Looks great! Thanks 🚀