Setup Ceremony: Implement phase connections and integrate with the rest of the codebase

[cronokirby]

Part of #712.

After doing the contributions in both phases, we should also connect the output of phase 1 with the start of phase 2, using the circuits.

This would also be a good occasion to test integration with the rest of our proving infrastructure, since we'll need to be interacting with our circuit format, among other things.

A good test here could be to try and replace our proving / verifying key generation code with code that calls out to the setup ceremony internals?

[cronokirby]

This can probably done as soon as #2689 is.