Replace Vec<Ascii> and [Ascii] with newtypes

The ascii crate now provides types for owned ascii strings and borrowed
ascii slices. Their implementation should allow them to be used like
`String` and `str`.

Changes:
 - `AsciiString` replaces `Vec<Ascii>`
 - `AsciiStr` replaces `[Ascii]`

Migration:
 - You can't construct `AsciiString` and `AsciiStr` like a collection
   anymore, use the `from_bytes` method instead.
 - To convert from the ascii types to bytes or strings please use the
   new generic conversion traits `Into` and `AsRef`.
 - The `AsciiCast` and `OwnedAsciiCast` traits are unchanged for now.

Implementations:
 - `AsciiString::from_bytes` and `AsciiStr::from_bytes` for construction from
   types which can be represented as a byte slice.
 - Implements `fmt::{Debug, Display}` like `String` does
 - `AsciiString` derefs to `AsciiStr`, additionally `DerefMut` is implemented so
   `AsciiExt` is fully useable
 - Implements `Hash`, `Borrow` and `ToOwned` so it can easily be used in
   collections.
 - Implements `PartialEq`, `PartialOrd`, `Eq` and `Ord` for itself and
   `str`
 - `AsciiString` implements `Into<String>`, `Into<Vec<u8>>` and
   `AsRef<AsciiStr>`
 - `AsciiStr` implements `AsRef<[u8]>` and `AsRef<str>`

[breaking-change]