Clean up GCD API (#923)

This mostly avoids exposing the separate `safegcd`/`bingcd`
implementations. Both use the same pre-processing for inputs so the
active variant only needs to be selected in `Odd<Uint>`. The
`safegcd(_vartime)` and `bingcd(_vartime)` methods are only exposed for
benchmarking at the moment.

Because a `const gcd` method on each integer type would have a higher
priority than the trait method, it would produce type errors when trying
to compute the GCD between different types (like `Uint` and `Int`). I've
gone with the name `gcd_uint` for these methods.

---------

Signed-off-by: Andrew Whitehead <[email protected]>

Author: andrewwhitehead

benches/uint.rs

@@ -427,23 +427,42 @@ fn gcd_bench<const LIMBS: usize>(g: &mut BenchmarkGroup<WallTime>, _x: Uint<LIMB
             BatchSize::SmallInput,
         )
     });
+
+    g.bench_function(BenchmarkId::new("safegcd", LIMBS), |b| {
+        b.iter_batched(
+            || {
+                (
+                    OddUint::<LIMBS>::random(&mut rng),
+                    Uint::<LIMBS>::random(&mut rng),
+                )
+            },
+            |(f, g)| black_box(f.safegcd(&g)),
+            BatchSize::SmallInput,
+        )
+    });
+
+    g.bench_function(BenchmarkId::new("safegcd_vartime", LIMBS), |b| {
+        b.iter_batched(
+            || {
+                (
+                    OddUint::<LIMBS>::random(&mut rng),
+                    Uint::<LIMBS>::random(&mut rng),
+                )
+            },
+            |(f, g)| black_box(f.safegcd_vartime(&g)),
+            BatchSize::SmallInput,
+        )
+    });
 }
 
 fn bench_gcd(c: &mut Criterion) {
     let mut group = c.benchmark_group("greatest common divisor");
 
     gcd_bench(&mut group, Uint::<1>::ZERO);
     gcd_bench(&mut group, Uint::<2>::ZERO);
-    gcd_bench(&mut group, Uint::<3>::ZERO);
     gcd_bench(&mut group, Uint::<4>::ZERO);
-    gcd_bench(&mut group, Uint::<5>::ZERO);
-    gcd_bench(&mut group, Uint::<6>::ZERO);
-    gcd_bench(&mut group, Uint::<7>::ZERO);
     gcd_bench(&mut group, Uint::<8>::ZERO);
-    gcd_bench(&mut group, Uint::<16>::ZERO);
-    gcd_bench(&mut group, Uint::<32>::ZERO);
     gcd_bench(&mut group, Uint::<64>::ZERO);
-    gcd_bench(&mut group, Uint::<128>::ZERO);
     gcd_bench(&mut group, Uint::<256>::ZERO);
 
     group.finish();