title: How do I get the cartesian product of 2 vectors by using Iterator?
body: <p>I have 2 <code>Vec</code>s:</p>
  <pre><code>let x = vec!['1', '2', '3'];
  let y = vec!['a', 'b', 'c'];
  </code></pre>
  <p>Now I want to use iterator to make a new vec like this <code>['1a', '1b', '1c', '2a', '2b', '2c', '3a', '3b', '3c']</code>. How can I do?</p>