slightly streamline array/array comparison

The net effect of this is to not recompute "the current
value of the first array" on every loop, pretty much.
However, the swap to make sure the inner loop is on the
longer array seems to be significant for performance.
On my system, this moves runtime from ~29us per op
to ~17us per op.

roaring/roaring.go

@@ -1903,16 +1903,26 @@ func intersectionCount(a, b *Container) int32 {
 
 func intersectionCountArrayArray(a, b *Container) (n int32) {
 	statsHit("intersectionCount/ArrayArray")
-	na, nb := len(a.array), len(b.array)
-	for i, j := 0, 0; i < na && j < nb; {
-		va, vb := a.array[i], b.array[j]
-		if va < vb {
-			i++
-		} else if va > vb {
-			j++
-		} else {
+	s1, s2 := a.array, b.array
+	if len(s1) == 0 || len(s2) == 0 {
+		return 0
+	}
+	if len(s1) > len(s2) {
+		s1, s2 = s2, s1
+	}
+	l2 := len(s2)
+	i2 := 0
+	v2 := s2[0]
+	for _, v1 := range s1 {
+		for v2 < v1 {
+			i2++
+			if i2 >= l2 {
+				return n
+			}
+			v2 = s2[i2]
+		}
+		if v2 == v1 {
 			n++
-			i, j = i+1, j+1
 		}
 	}
 	return n