Add APIs for dealing with titlecase

[Jules-Bertholet]

ACP: rust-lang/libs-team#354

r? libs-api

@rustbot label 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.

[bors]

☔ The latest upstream changes (presumably #122616) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #122013) made this pull request unmergeable. Please resolve the merge conflicts.

[rustbot]

library/core/src/unicode/unicode_data.rs is generated by the src/tools/unicode-table-generator tool.

If you want to modify unicode_data.rs, please modify the tool then regenerate the library source file via ./x run src/tools/unicode-table-generator instead of editing unicode_data.rs manually.

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[Jules-Bertholet]

The libs team gave a positive response to the ACP. @rustbot label -S-waiting-on-ACP S-waiting-on-review

[rust-bors]

☔ The latest upstream changes (presumably #152574) made this pull request unmergeable. Please resolve the merge conflicts.