`algorithms/gcd/matrix`: Fix the below method to get double-word Lehmer GCD.

*On 2022-06-06 @recmo wrote in [`4c74c09`](https://github.com/recmo/uint/commit/4c74c09b4d6f425cda931e9f5e8d00f1367c4261) “Merge pull request #110 from recmo/gcd”:*

Fix the below method to get double-word Lehmer GCD.

```rust
if q == Matrix::IDENTITY {
return q;
}
// We can return q here and have a perfectly valid single-word Lehmer GCD.
q
// OPT: Fix the below method to get double-word Lehmer GCD.

// Recompute r0 and r1 and take the high bits.
// TODO: Is it safe to do this based on just the u128 prefix?
// let (r0, r1) = q.apply_u128(r0, r1);
// let s = r0.leading_zeros();

```
*From [`src/algorithms/gcd/matrix.rs:312`](https://github.com/recmo/uint/blob/4c74c09b4d6f425cda931e9f5e8d00f1367c4261/src/algorithms/gcd/matrix.rs#L312)*