Function verify_respondent 

pub fn verify_respondent(
    q: &BigUint,
    paillier: &PaillierPublicKey,
    ck: &CommitmentKey,
    c1: &BigUint,
    c2: &BigUint,
    proof: &RespondentProof,
) -> bool

Verify a responder proof binding c₂ to c₁ with bounded secrets.