Benchmark pow on BigUint (from #152)

benches/bigint.rs

@@ -188,3 +188,16 @@ fn hash(b: &mut Bencher) {
         assert_eq!(h.len(), v.len());
     });
 }
+
+#[bench]
+fn pow_bench(b: &mut Bencher) {
+    b.iter(|| {
+        let upper = 250_usize;
+        for i in 2..upper + 1 {
+            for j in 2..upper + 1 {
+                let i_big = BigUint::from_usize(i).unwrap();
+                num::pow(i_big, j);
+            }
+        }
+    });
+}