make Bound and Borrowed types public API

[davidhewitt]

This completes renaming of our temporary names Py2 and Py2Borrowed to Bound and Borrowed, and adds these plus the methods traits to the public API (and the prelude).

[adamreichold]

downcast_into_unchecked is now public yet missing a # Safety section.

[davidhewitt]

Added, sorry for the slight delay, had a few bits and pieces going on this afternoon.

[codspeed-hq]

CodSpeed Performance Report

Merging #3686 will improve performances by 22.03%

_{Comparing davidhewitt:bound (4ac6a6b) with main (2788f4a)}

Summary

⚡ 1 improvements
✅ 77 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	davidhewitt:bound	Change
⚡	not_a_list_via_downcast	153.9 ns	126.1 ns	+22.03%