Auto merge of #26039 - SimonSapin:case-mapping, r=alexcrichton

* Add “complex” mappings to `char::to_lowercase` and `char::to_uppercase`, making them yield sometimes more than on `char`: #25800. `str::to_lowercase` and `str::to_uppercase` are affected as well.
* Add `char::to_titlecase`, since it’s the same algorithm (just different data). However this does **not** add `str::to_titlecase`, as that would require UAX#29 Unicode Text Segmentation which we decided not to include in of `std`: https://github.com/rust-lang/rfcs/pull/1054 I made `char::to_titlecase` immediately `#[stable]`, since it’s so similar to `char::to_uppercase` that’s already stable. Let me know if it should be `#[unstable]` for a while.
* Add a special case for upper-case Sigma in word-final position in `str::to_lowercase`: #26035. This is the only language-independent conditional mapping currently in `SpecialCasing.txt`.
* Stabilize `str::to_lowercase` and `str::to_uppercase`. The `&self -> String` on `str` signature seems straightforward enough, and the only relevant issue I’ve found is #24536 about naming. But `char` already has stable methods with the same name, and deprecating them for a rename doesn’t seem worth it.

r? @alexcrichton 

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