0.5.2
Impl Deref for ByteString.

This lets you do `&*some_byte_string` to get a slice of it.

You can also do `some_byte_string[]` to get a slice, but it seems
that the future of `[]` is murky.
Renamed `as_utf8_string` to `into_utf8_string` because it takes
`self` by value.

[breaking-change]
0.6.0