[modules] Add a simple micro-benchmark for import/export accesses.

test/js-perf-test/Modules/basic-namespace.js

+// Copyright 2017 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import * as m from "value.js";
+for (let i = 0; i < iterations; ++i) m.set(m.value + 1);
+if (m.value != iterations) throw m.value;
+m.set(0);