Investigate if we could use serde for message serialisation

[ah-]

Currently the serialisation is a bunch of slightly sketchy mem transmutes and enums. It works ok, and is as fast as it gets, and produces little binary size overhead.

It would be interesting to see how it would look if we used serde in https://github.com/ah-/anne-key/blob/master/src/protocol.rs and e.g. https://github.com/ah-/anne-key/blob/master/src/bluetooth.rs#L69

[hdhoang]

reddit discussion brought up abomonation, though it looks to be not no_std.