Add a bench for bigint multiply

This bench allows to see the increase of time
if we double the size of one of the operands.

benches/bigint.rs

@@ -78,6 +78,11 @@ fn multiply_2(b: &mut Bencher) {
     multiply_bench(b, 1 << 16, 1 << 16);
 }
 
+#[bench]
+fn multiply_3(b: &mut Bencher) {
+    multiply_bench(b, 1 << 16, 1 << 17);
+}
+
 #[bench]
 fn divide_0(b: &mut Bencher) {
     divide_bench(b, 1 << 8, 1 << 6);