port IntoPyDict to Bound API

[Icxolu]

Part of #3684, split from #3716

This introduces the into_py_dict_bound API on the IntoPyDict trait, and deprecates the old gil-ref API.

Because the diff is already quite large, I kept the deprecation of PyDict::new for a followup PR.

[davidhewitt]

Thanks yet again, this looks great! Just one possible thought / suggestion...

[codspeed-hq]

CodSpeed Performance Report

Merging #3817 will degrade performances by 16.95%

_{Comparing Icxolu:into-pydict (e45fbe4) with main (45f2b0a)}

Summary

⚡ 1 improvements
❌ 1 regressions
✅ 77 untouched benchmarks

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

Benchmarks breakdown

	Benchmark	main	Icxolu:into-pydict	Change
❌	sequence_from_list	272.2 ns	327.8 ns	-16.95%
⚡	f64_from_pyobject	461.1 ns	405.6 ns	+13.7%