Interfaces for recursion, folding

[codygunton]

For one, we should rename verify_proof to aggregate or fold as the case may be.

With Plonk we have to specify a manifest. Probably Barretenberg should just expose some instantiations of recursion functions rather than exposing the flavor and asking the circuit writer to instantiate. Relatedly, it feels wrong that I needed to define a simulator flavor to simulate recursion (or, really, aggregation).