Rewrite extensions in Rust

[sdispater]

This PR is the first introduction to Rust in the codebase.

It is a complete rewrite of existing C extensions and paves the way to more speed improvements by rewriting other parts of the codebase in the future.

Note that if there is still a pure Python version of these extensions and will always be for the foreseeable future.

Be aware that this PR introduces small backwards incompatibilities when parsing ISO8601 strings:

• Basic time representation now requires a leading T, for instance T201205
• Fractional hours and minutes are no longer supported. It was technically supported before the behavior was incorrect so support has been removed for now but will be reintroduced in the future.

Pull Request Check List

• Added tests for changed code.
• Updated documentation for changed code.

[codspeed-hq]

CodSpeed Performance Report

Merging #721 will degrade performances by 18.61%

_{Comparing refactor/rust (d29ce1b) with master (06fb2e9)}

Summary

❌ 1 regressions
✅ 1 untouched benchmarks

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

Benchmarks breakdown

	Benchmark	master	refactor/rust	Change
❌	test_format	2.3 ms	2.8 ms	-18.61%

[Secrus]

Some issues here and there, but overall, nice job. I am no Rust expert, so I just looked through it trusting that the test suite checks if it works correctly.

[Maneren]

Great initiative, seeing C rewritten to Rust always makes me happy. I suggested some minor refactorings (partially suggested by clippy::pedantic) but otherwise very good job. There are still few noticeable remnants of C that don't really fit into Rust (eg. the null char) but they require more complex changes and it isn't really necessary to do that.

Also, I am quite new to reviewing (got here fairly randomly) so sorry if there's anything wrong in that regard.

[sdispater]

Thanks for the thorough review @Maneren! I applied all of your suggestions.

[Maneren]

Happy to help

[Secrus]

Few extra nitpicks

[Secrus]

LGTM, great job!