DecommitVerify receives a `Proof` and a `Commitment`; it first checks that the proof actually opens the commitment; then it verifies the proof.

NewProver generates a `Prover` object, ready to generate Schnorr proofs on any given point.

Verify verifies the `proof`, given the prover parameters `scalar` and `curve`.

Proof contains the (c, s) schnorr proof.