Add iter_n

Author: Manishearth

text/0000-benchmarking.md

@@ -107,7 +107,16 @@ fn my_benchmark(bench: Bencher) -> BenchResult {
     bench.iter(|| {
         black_box(pow(y, x));
         pow(x, y)
-    });
+    })
+}
+```
+
+In case you want the benchmark to run for a predetermined number of times, use `iter_n`:
+
+```rust
+#[bench]
+fn my_benchmark(bench: Bencher) -> BenchResult {
+    bench.iter_n(1000, || do_some_stuff());
 }
 ```