title: element-wise concatenation of string vectors
body: <p>Say I have two character vectors:</p>
  
  <pre><code>a &lt;- c("a", "b", "c")
  b &lt;- c("1", "2", "3")
  </code></pre>
  
  <p>How do I merge them such that I get:</p>
  
  <pre><code>ab &lt;- c("a1", "b2", "c3")
  </code></pre>