message: use wrapping add instead of casting
diff: @@ -693,8 +693,8 @@ fn add_256bits_v1(x: &[u8], y: &[u8]) -> [u8; 32] {
  let mut out = [0u8; 32];
  for i in 0..32 {
  - let r = x[i] as u16 + y[i] as u16;
  - out[i] = r as u8;
  + let r = x[i].wrapping_add(y[i]);
  + out[i] = r;
  }
  out
  }
  