`algorithms/mul_redc`: Create cmp routine

[github-actions[bot]]

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

Create cmp routine

    // Copy result.
    result.copy_from_slice(&temp[m.len()..2 * m.len()]);

    // Subtract one more m if result >= m
    let mut reduce = true;
    // REFACTOR: Create cmp routine
    if temp[temp.len() - 1] == 0 {
        for (r, m) in zip(result.iter().rev(), m.iter().rev()) {
            if r < m {
                reduce = false;
                break;

From src/algorithms/mul_redc.rs:44