Reduce boilerplate when merging rounds

[fjarri]

The merged KeyGen + KeyRefresh protocol requires us to write quite a bit of boilerplate where all we do is just merge two types into one or separate them to pass on to lower levels. Can it be done generically?

Seems like we can't do it via a trait, the blanket impl will conflict with the sequential wrapper. Also for the parallel merge we need to assert that round numbers, destinations etc are the same between the two rounds. So a macro would probably be better.

[fjarri]

The discussion is moved to entropyxyz/manul#3