message: be explicit about overflows
diff: @@ -190,7 +190,7 @@ pub fn chacha20_rows<F: FftField>(s0: Vec<u32>) -> Vec<Vec<F>> {
  let nyb = |t: u32, i: usize| f((t >> (4 * i)) & 0b1111);
  let top_bit = (((s[x] as u64) + (s[z] as u64)) >> 32) as u32;
  - let xprime = s[x] + s[z];
  + let xprime = u32::wrapping_add(s[x], s[z]);
  let y_xor_xprime = s[y] ^ xprime;
  let yprime = y_xor_xprime.rotate_left(k);
  @@ -255,7 +255,7 @@ pub fn chacha20_rows<F: FftField>(s0: Vec<u32>) -> Vec<Vec<F>> {
  pub fn chacha20(mut s: Vec<u32>) -> Vec<u32> {
  let mut line = |x, y, z, k| {
  - s[x] += s[z];
  + s[x] = u32::wrapping_add(s[x], s[z]);
  s[y] ^= s[x];
  let yy : u32 = s[y];
  s[y] = yy.rotate_left(k);
  