add PyBackedStr and PyBackedBytes

[davidhewitt]

Split from #3708

cc @adamreichold this is inspired by #3784 (comment)

I didn't really want to think too hard about a generic T for this type, instead I wanted to just introduce two special cases which should be able to replace &str or Cow<u8>. These types might end up being temporary just until we can add the second lifetime to FromPyObject, or maybe we learn from these and refine further later.

This depends on #3801 to compile.

[codspeed-hq]

CodSpeed Performance Report

Merging #3802 will degrade performances by 20.41%

_{Comparing davidhewitt:backed-str (6aec91c) with main (f3ddd02)}

Summary

⚡ 1 improvements
❌ 2 regressions
✅ 76 untouched benchmarks

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

Benchmarks breakdown

	Benchmark	main	davidhewitt:backed-str	Change
❌	list_via_downcast	157.2 ns	185 ns	-15.02%
⚡	list_via_extract	364.4 ns	308.9 ns	+17.99%
❌	not_a_list_via_downcast	216.7 ns	272.2 ns	-20.41%

[martindurant]

Thank you, this is exactly the kind of code I have used and I see elsewhere for zero-copy of python bytes. What remains to be done in this PR?

(I care more about bytes than strings, so would be happy to see this without the UTF8 encode)