Const str methods

[KGrewal1]

First PR for libs, so may be doing stuff entirely wrong - please correct me!

Make str::split_at_mut_unchecked const for consistency with slice::split_at_mut_unchecked and introduce str::split_at_unchecked, changing the internals of str::split_at_checked to use it

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @Noratrieb (or someone else) some time within the next two weeks.

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[bors]

☔ The latest upstream changes (presumably #132317) made this pull request unmergeable. Please resolve the merge conflicts.

[KGrewal1]

Ah did not realise there was a PR for this already