Automate unmarshaling

[stv0g]

By @koraa in #27 (comment)

https://github.com/stv0g/go-rosenpass/blob/d7e38ecaf9e7803f2824a03ac24ac34944a53af6/messages.go#L184-201

Severity: B – Manual offset arithmetic like that is treacherous. There is a danger of subtle errors.
In the Rust implementation we put a lot of work into writing a macro that automatically generates this code.
Would it be possible to do something similar in rust?