message: Improve mac_digit bounds checking
  By starting with `split_at_mut`, the hot multiplication loop runs with
  no bounds checking at all!  The remaining carry loop has a slightly
  simpler check for when the remaining iterator runs dry.
diff: @@ -227,16 +227,16 @@ fn mac_digit(acc: &mut [BigDigit], b: &[BigDigit], c: BigDigit) {
  }
  let mut carry = 0;
  + let (a_lo, a_hi) = acc.split_at_mut(b.len());
  - for (i, bi) in b.iter().enumerate() {
  - acc[i] = mac_with_carry(acc[i], *bi, c, &mut carry);
  + for (a, &b) in a_lo.iter_mut().zip(b) {
  + *a = mac_with_carry(*a, b, c, &mut carry);
  }
  - let mut i = b.len();
  -
  + let mut a = a_hi.iter_mut();
  while carry != 0 {
  - acc[i] = adc(acc[i], 0, &mut carry);
  - i += 1;
  + let a = a.next().expect("carry overflow during multiplication!");
  + *a = adc(*a, 0, &mut carry);
  }
  }
  