std::ascii needs some improvements

[Kimundi]

• (~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.