Bench sub.

Improve performance for sub.

benches/bench.rs

@@ -33,11 +33,20 @@ fn insert(data: &[u64]) {
     }
 }
 
+fn sub(a: &BitSet, b: &BitSet) {
+    a - b;
+}
+
 fn criterion_benchmark(c: &mut Criterion) {
     let data: Vec<_> = default_set(1000);
 
     c.bench_function("contains", |b| b.iter(|| contains(&data)));
     c.bench_function("insert", |b| b.iter(|| insert(&data)));
+
+    let lhs: BitSet = default_set(1000);
+    let rhs: BitSet = default_set(1000);
+
+    c.bench_function("sub", |b| b.iter(|| sub(&lhs, &rhs)));
 }
 
 criterion_group!(benches, criterion_benchmark);