Add state machine

[survived]

State machine wraps an MPC protocol defined as async function and provides sync interface to carry it out

Blocked by #6

[maurges]

std::mem::replace(&mut s.yielded, false). But it really doesn't matter, feel free to ignore

[survived]

I'll try this next time ;)

[maurges]

You know what, I can't think about this for two more weeks, let's see if we get some real bugs in real usage