Investiage inversion of control

[fjarri]

Writing down the protocol in terms of separate rounds is flexible, but a little unwieldy. Can we provide a way to write protocols as a single function, without splitting it into Round types? Kind of like in round-based.

This probably won't support all the functionality Rounds do, so the question is: what can we support? Is it a useful subset? Can it be linked to the Round-based description so that it's easy to prototype a protocol as a single function and later switch to Rounds?