Implement int_from_ascii_radix

[niklasf]

Motivation here is to allow parsing integers without UTF-8 validation. Useful when strings are UTF-8 by only convention, and even some binary protocols. Similar to #101035.

Behaves exactly like from_str_radix, which already works with raw bytes internally.

[rustbot]

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[niklasf]

@rustbot label +T-libs-api -T-libs

[Mark-Simulacrum]

This will need an accompanying library API change proposal as the bot indicates:

If this PR contains changes to any unstable APIs please edit the PR description to add a link to the relevant API Change Proposal or create one if you haven't already.

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[niklasf]

Thanks! There are some open questions regarding the + character, which makes sense for FromStr, but may be unexpected in binary protocols. Abandoning this PR for now.