std::ascii needs some improvements

- `(~str).to_ascii_consume` is incorrect, lacks tests.
- Need a way to go `~[Ascii] -> ~[u8]` (both consuming/unconsuming).
- Naming conventions could be changed: `as_` prefix for cheap casts, `to_` as prefix for copying to a new one, `into_` for moving into a new type.
- Unsafe versions without safety checks.
