`algorithms/gcd/mod`: Compute using absolute value instead of patching sign.

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

Compute using absolute value instead of patching sign.

```rust
m.apply(&mut s0, &mut s1);
m.apply(&mut t0, &mut t1);
even ^= !m.4;
}
}
// TODO: Compute using absolute value instead of patching sign.
if even {
// t negative
t0 = Uint::ZERO - t0;
} else {
// s negative

```
*From [`src/algorithms/gcd/mod.rs:107`](https://github.com/recmo/uint/blob/4c74c09b4d6f425cda931e9f5e8d00f1367c4261/src/algorithms/gcd/mod.rs#L107)*