Implement the verification gadget

[nikkolasg]

For testudo to deliver a compact proof, we need to have a circuit that verifies a testudo proof. We could use this circuit with Groth16 to deliver a short verification time and proof size.
This issue depends first on #36 .
Then it requires to implement:

• Groth16 verifier in circuit (using arkworks it's already implemented)
• MIPP verifier (inner pairing product argument)
• PST verifier