Implement DerefMut for String, enabling make_ascii_*case

[SimonSapin]

The AsciiExt trait has make_ascii_uppercase and make_ascii_lowercase methods that take &mut self. This trait is implemented for str, but there is currently no easy way to get a &mut str value.

&mut str is rarely useful, but ASCII case mapping is one of the rare cases. The intent (see rust-lang/rust#22024 (comment)) was that String would implement DerefMut, making these methods usable on it through deref to &mut str. But that last step has not been done yet. Is it time to do it? Is there a reason not to?

CC @alexcrichton @aturon

[Gankra]

We should do it.

[alexcrichton]

I don't think there's a reason to not do it, it just hasn't been done. This is pretty minor though so I'm gonna close this (no need to have a whole issue tracking this).