Add mod std::string::raw

std::string::raw has one function, from_utf(), that converts a Vec<u8>
to a String without testing for valid UTF-8.
Rename various things in std::ascii

Rename a few traits in std::ascii to have more descriptive names. Also
reimplement a few methods to avoid unnecessary allocation. And add a
trait for Vec<Ascii> to do things like casemapping in-place.

- Rename AsciiCast to ToAscii
- Rename OwnedAsciiCast to IntoAscii
- Split AsciiStr into two traits, the second called AsciiSlice
- Add trait AsciiVec
- Rename OwnedStrAsciiExt to StringAsciiExt

This just barely qualifies as a breaking change. Every trait except
StringAsciiExt is in the prelude.

[breaking-change]
Rename more std::ascii methods for consistency

Rename the various *_lower and *_upper methods to the equivalent
*_lowercase and *_uppercase names.

[breaking-change]
Update the various clients of ascii for the new names

This includes updating for the previous rename of to_lower/upper on
Ascii.