Overview
Add implementations for verify_groth16 and prove_groth16 to the MockProver. These implementations simply check that the public_values and vkey_hash are correct.

This ensures that the following user flow is correct:

1. User generates a proof in mock mode and saves it locally.
2. User loads in the proof and calls verify_groth16 on it in mock mode. If they are using the correct vkey (i.e. their program hasn't changed), then the proof will verify.