set、map、for-of和迭代器

set：
https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Set

map:
https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Map

for-of和迭代器:
http://www.youkuaiyun.com/article/2015-06-15/2824965-es6-in-depth-iterators-and-the-for-of-loop