TEST: Add bench for .sort_by()

Author: bluss

benches/bench.rs

@@ -643,3 +643,14 @@ fn many_retain_hashmap_100_000(b: &mut Bencher) {
         map
     });
 }
+
+#[bench]
+fn ordermap_sort(b: &mut Bencher) {
+    let map = OMAP_100K.clone();
+
+    // there's a map clone there, but it's still useful to profile this
+    b.iter(|| {
+        let mut map = map.clone();
+        map.sort_keys();
+    });
+}