Rollup merge of #131520 - zachs18:const-str-split, r=Noratrieb

Mark `str::is_char_boundary` and `str::split_at*` unstably `const`.

Tracking issues: #131516, #131518

First commit implements `const_is_char_boundary`, second commit implements `const_str_split_at` (which depends on `const_is_char_boundary`)

~~I used `const_eval_select` for `is_char_boundary` since there is a comment about optimizations that would theoretically not happen with the simple `const`-compatible version (since `slice::get` is not `const`ifiable) cc #84751. I have not checked if this code difference is still required for the optimization, so it might not be worth the code complication, but 🤷.~~

This changes `str::split_at_checked` to use a new private helper function `split_at_unchecked` (copied from `split_at_mut_unchecked`) that does pointer stuff instead of `get_unchecked`, since that is not currently `const`ifiable due to using the `SliceIndex` trait.

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