Escape control

[LukasKalbertodt]

Add escape_control to char and str: Escapes C0 and C1 control chars. When printing chars or strs via {:?} the Debug-implementation will use those methods instead of escape_default.
Fixes #22412 (?)

This is my first PR, I hope everything is ok...

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @nikomatsakis (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. The way Github handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see CONTRIBUTING.md for more information.

[eddyb]

Accidentally removed trailing newline.

[alexcrichton]

Thanks for the PR! At this stage of Rust, however, all new APIs require an RFC for inclusion into the standard library. New APIs also start out as #[unstable] after being accepted via an RFC and only later will they be maked as #[stable] (typically at least a release cycle).

For moving forward here this would need to open an RFC for the addition of these APIs. Feel free to let me know if you need any help writing an RFC, I'd be more than willing to review any drafts!