Remove proof_gen crate

[Nashtare]

This was actually simpler / quicker than expected, probably could have been grouped with #649.

Motivation:

• constants module unused
• ProverStateBuilder / VerifierStateBuilder are not used
• redundant calls for the proving methods that could be done directly in the ops module of zero
• custom Result type that was just wrapping errors into Strings

Also renames prove_transaction_aggregation into prove_batch_aggregation for consistency with the chosen names in zero/ (as we don't prove individual transactions anymore but batches).

[0xaatif]

I love this!