Move change to protocol

[alexjg]

This moves automerge_backend::UnencodedChange to 'automerge_protocol::UncompressedChange. Unfortunately this change touches a lot of files. The primary meaningful difference is that the UnencodedChangereferred toautomerge_backend::Operationwhereas theUncompressedChangeusesautomerge_frontend::Op. This neccessitates a fallible conversion from UncompressedChangetoautomerge_backend::Change, which is implemented here as impl TryFrom for Changeinchange.rs, which replaces Change::decode`.