Implement Message for uuid::Uuid

[axyiee]

Pull Request Information

Title is self-explanatory.

• Closes Example of docs for extern_path doesn't compile #568

[neoeinstein]

We generally have not elected to take optional implementations like this into Prost. This is in part because the implementation you propose here, while one way to do it, is not necessarily the one cannonical form that we should bless. (The Protobuf specification does not describe a canonical way to encode/decode a UUID.) Some users out there are likely encoding UUIDs as strings, while others are encoding them, as you have, by using bytes.

In this case, this implementation is likely better left to a newtype wrapper that does what you are looking for, rather than being an integral part of either uuid or prost.

[LucioFranco]

Right the correct solution is for either uuid to provide the Message impl (like it does for serde) or for you to do a newtype warpper and implement it yourself.