message: use std::mem to call mem::swap
diff: +use std::mem;
  use std::ops::Shl;
  use traits::{One, Zero};
  @@ -190,7 +191,7 @@ pub fn monty_modpow(x: &BigUint, y: &BigUint, m: &BigUint) -> BigUint {
  mr.n0inv,
  num_words,
  );
  - ::std::mem::swap(&mut z, &mut zz);
  + mem::swap(&mut z, &mut zz);
  yi <<= n;
  j += n;
  }
  