algorithms/gcd/gcd_old: Would this be faster using extended binary gcd?

[github-actions]

On 2022-06-06 @recmo wrote in 4c74c09 “Merge pull request #110 from recmo/gcd”:

Would this be faster using extended binary gcd?
We shadow q for readability.

}

/// Compute the Lehmer update matrix for small values.
///
/// This is essentialy Euclids extended GCD algorithm for 64 bits.
// OPT: Would this be faster using extended binary gcd?
// We shadow q for readability.
#[allow(clippy::shadow_unrelated)]
fn lehmer_small(mut r0: u64, mut r1: u64) -> Matrix {
    debug_assert!(r0 >= r1);
    if r1 == 0_u64 {
        return Matrix::IDENTITY;

From src/algorithms/gcd/gcd_old.rs:126