Add TryExtend trait

[bschoenmaeckers]

This PR adds a TryExtend trait that is similar to the std::iter::Extend trait except it is failable. Currently implemented for PyList & PyDict, including some optimised specializations on nightly.

[bschoenmaeckers]

I could not find a obvious file to add this trait to so I created a new one. Suggestions are welcome.

[mejrs]

I'm unsure whether I'm in favor of this, but I don't want specialization. We used to rely on it but removed it a couple of years ago. The problem with specialization (even the min version) is that it has soundness issues when it comes to lifetimes and it also has no realistic path towards stabilization.

Also, you can add a compatibility shim for PyList_Extend in the compat module.

[bschoenmaeckers]

Alternatively I could add 2 methods to PyList & PyDict; extend_from_pyobject and extend_from_iter. This does not require specialization and is optimized for both cases.

[codspeed-hq]

CodSpeed Performance Report

Merging #4664 will improve performances by 16.34%

_{Comparing bschoenmaeckers:try-extend (06d8289) with main (2d3bdc0)}

Summary

⚡ 5 improvements
✅ 78 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	bschoenmaeckers:try-extend	Change
⚡	test_empty_class_init_py	15.3 µs	13.8 µs	+10.85%
⚡	test_args_kwargs_py	21.1 µs	18.5 µs	+14.27%
⚡	test_args_kwargs_rs	15.8 µs	13.6 µs	+16.34%
⚡	test_simple_kwargs_py	22.3 µs	19.9 µs	+12.46%
⚡	test_simple_kwargs_rs	21.6 µs	18.7 µs	+15.28%