message: make csprng non mut
diff: @@ -88,7 +88,7 @@ pub fn evaluate(
  slotid: SlotId,
  ) -> Option<Witness> {
  let input = Input::create(nonce, slotid);
  - let mut csprng = OsRng::new().unwrap();
  + let csprng = OsRng::new().unwrap();
  let vr = vrf_evaluate_and_proove(key, &input.0, csprng);
  let r = vrf_verified_get_output::<Curve25519_2HashDH>(&vr);
  let t = get_threshold(&input, &r);
  