This changes the deserialize implementation for a borrowed Cow<[u8]>
to specifically request a byte slice, rather than a borrowed string.
The old behavior breaks any program which relies on data being
deserialized the same way as it was serialized and uses Cow<[u8]>.
In serde_json, it just wouldn't deserialize. In bincode, it
deserialized normally unless the bytes were invalid UTF8.
Fixes https://github.com/TyOverby/bincode/issues/231.
