`algorithms/mul_redc`: Create add_mul1 routine.

[github-actions[bot]]

On 2022-06-09 @recmo wrote in f0f0ddb “Merge pull request #126 from recmo/redc”:

Create add_mul1 routine.

    // Reduce temp.
    for i in 0..m.len() {
        let u = temp[i].wrapping_mul(inv);

        // REFACTOR: Create add_mul1 routine.
        let mut carry = 0;
        #[allow(clippy::cast_possible_truncation)] // Intentional
        for j in 0..m.len() {
            carry += u128::from(temp[i + j]) + u128::from(m[j]) * u128::from(u);
            temp[i + j] = carry as u64;

From src/algorithms/mul_redc.rs:21