add PyString::new_bound

[davidhewitt]

Part of #3684

This implements PyString::new_bound and deprecates PyString::new as per the established pattern, updating internal code.

There are other constructors on PyString, particularly PyString::intern, which I will cover in a separate PR to keep the diff smaller.

[davidhewitt]

Sorry for the force-pushes here; I'll try to make the next one I push a bit tidier on first push... 🤦

[codspeed-hq]

CodSpeed Performance Report

Merging #3774 will degrade performances by 29.03%

_{Comparing davidhewitt:string-new-bound (c475656) with main (7549a21)}

Summary

❌ 2 regressions
✅ 78 untouched benchmarks

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

Benchmarks breakdown

	Benchmark	main	davidhewitt:string-new-bound	Change
❌	extract_float_downcast_success	418.9 ns	474.4 ns	-11.71%
❌	extract_str_extract_success	675 ns	951.1 ns	-29.03%

[adamreichold]

This will have to wait for the corresponding PyErr change, right?

[davidhewitt]

Yes, we need to add PyErr::from_value_bound I think.

(or add a trait which abstracts over &PyAny and &Bound<PyAny> just temporarily to ease migration, thought that'd be a new design question and less mechanical.)