Define top-level message types

[jschneider-bensch]

This PR introduces the message types required in the top-level MPC protocol (i.e. subprotocol message types are not yet defined).

I also define some basic data structures which make up the message payloads.

[keks]

This looks good! We just have to make sure that the #[allow(dead_code)] annotation will get deleted, but this repo is small enough that this will be caught.