Auto merge of #26561 - SimonSapin:remove-titlecase, r=alexcrichton

I added it because it was easy (same a `char::to_lowercase`, just a different table), but it doesn’t make sense to have this in std but not str::to_titlecase, which would require https://github.com/unicode-rs/unicode-segmentation

At some point in the future this feature will be available (both on char and str) in a crates.io crate.