Make {str,[u8]}::eq_ignore_ascii_case const

[coolreader18]

Both of these can trivially be implemented with already-const-stable language constructs/functions, so I figured it would make sense to make them const. I was going to make make_ascii_{upper,lower}case const too, but I realized that requires const_mut_refs and u8/char::make_ascii_*case aren't even unstably const under that, so I figured I'd leave that for after that's stabilized. Also, unfortunately, neither of the existing implementations can just have const added (maybe is_ascii, but it'd need a bunch of extra unstably-const ptr methods), and also eq_ignore_ascii_case codegen'd worse with the const-friendly while loop version, so I had to use const_eval_select for both.

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[coolreader18]

Failure was just that I forgot to add SAFETY comments for the unsafe blocks.

[bors]

☔ The latest upstream changes (presumably #102795) made this pull request unmergeable. Please resolve the merge conflicts.

[ChrisDenton]

Not much movement on this yet so cc @rust-lang/wg-const-eval in case anyone has any thoughts on this.

[fee1-dead]

I think this is okay (because it doesn't use const traits) but I think libs-api should have a say on whether use of const_eval_select is okay in terms of readability and maintainability.

[bors]

☔ The latest upstream changes (presumably #111113) made this pull request unmergeable. Please resolve the merge conflicts.

[coolreader18]

Actually, I really don't feel strongly about just eq_ignore_ascii_case (especially since slice == isn't even const stable yet) so I'll close this since #111090 did is_ascii

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