Implement `Display` for structs in std::char

[clarfonthey]

Right now, EscapeUnicode, EscapeDefault, EscapeDebug, ToLowercase, and ToUppercase do not implement Display. I think that this would be rather useful, because then you could simplyprint!("{}", 'ß'.to_uppercase()) instead of having to do a for loop or convert to a String (which is costly).

Code is implemented in rust-lang/rust#38909.

[clarfonthey]

Closing this because a pull request will suffice.