Add an extend combinator for protocols.

[fjarri]

The combinator will allow building on top of an existing protocol, adding features. This is useful e.g. for extending AuxGen to also do KeyRefresh in CGGMP. KeyRefresh needs some intermediate information created during AuxGen (Paillier keys), so the new protocol would have to introspect the AuxGen state, add information to the outgoing messages, extract a part of information from the received messages, and add logic to finalization too.

This scenario is a superset of Misbehave from #60, but the question is, if we implement it that way, will it add any boilerplate to the usage in tests? If it does, we better keep them separate.