message: bigint::monty: simplify modpow zero test
diff: use integer::Integer;
  -use traits::{Zero, One};
  +use traits::Zero;
  use biguint::BigUint;
  @@ -116,8 +116,7 @@ pub fn monty_modpow(a: &BigUint, exp: &BigUint, modulus: &BigUint) -> BigUint{
  // Binary exponentiation
  let mut ans = &r % mr.p;
  let mut e = exp.clone();
  - let zero = Zero::zero();
  - while e > zero {
  + while !e.is_zero() {
  if e.is_odd() {
  ans = monty_mult(ans, &apri, &mr);
  }
  