Differentiate between [u8] and binary.

[mneumann]

[u8] will be encoded as arrays of integers, which is not very efficient as every byte potentially needs two bytes. It would be better to encode it as a msgpack binary (when supported). But with the current Decodable trait this seems to be not possible as we cannot specialize code for a specific element type.

A solution is to introduce a type binary = [u8].