Add a zip combinator for protocols

[fjarri]

This is the scenario 3 from #60: executing two independent protocols simultaneously by bundling their messages and splitting the incoming messages between two protocols. This is useful e.g. for merging KeyInit and AuxGen in CGGMP.

The protocols don't have to have the same number of rounds, as soon as one finishes we can just hold on to the result until the other finishes too.