Encoding/Decoding bytestrings with libserialize

[aochagavia]

Some encoding protocols don't enforce UTF8 in strings. In this cases, it would be useful to provide functions to encode/decode a string as an array of bytes (maybe something like read_bytestring and emit_bytestring).

An example of this situation is the bencode protocol. Currently, the only way to decode a struct containig a byte string is to implement a special trait called FromBencode.

Another use case for this is to (de)serialize XML (see this subreddit thread)

Should I submit a PR to add this?

[mneumann]

related to #12293, and needed for msgpack as well.

[steveklabnik]

Serialize has now been moved out of tree.