Implement DerefMut for String, enabling make_ascii_*case #1157

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