message: Remove redundant signature check
  E::Fs guarantees its representation is in the field, implicitly enforcing
  that S < order(G).
diff: @@ -148,16 +148,18 @@ impl<E: JubjubEngine> PublicKey<E> {
  // c = H*(Rbar || M)
  let c = h_star::<E>(&sig.rbar[..], msg);
  + // Signature checks:
  + // R != invalid
  let r = match Point::read(&sig.rbar[..], params) {
  Ok(r) => r,
  Err(_) => return false,
  };
  + // S < order(G)
  + // (E::Fs guarantees its representation is in the field)
  let s = match read_scalar::<E, &[u8]>(&sig.sbar[..]) {
  Ok(s) => s,
  Err(_) => return false,
  };
  - // S < order(G)
  - s.into_repr() < E::Fs::char() &&
  // S . P_G = R + c . vk
  self.0.mul(c, params).add(&r, params) == params.generator(p_g).mul(s, params).into()
  }
  