Rollup merge of #122668 - Jules-Bertholet:titlecase, r=Mark-Simulacrum

Add APIs for dealing with titlecase

ACP: https://github.com/rust-lang/libs-team/issues/354
Tracking issue: https://github.com/rust-lang/rust/issues/153892

r? libs-api

@rustbot label T-libs -T-libs-api A-unicode

~~The last commit has some insta-stable `PartialEq` impls, therefore: @rustbot label -needs-fcp
Alternatively, I could split those out into a follow-up PR.~~ (Edit: will do in follow-up)