title: Crystal: how to copy String::Builder s to each other
body: <p>I want to write contents on one String::Builder to another, like:</p>
  
  <pre><code>str1 = String::Builder.new
  str2 = String::Builder.new
  
  str1 &lt;&lt; "foo"
  str2 &lt;&lt; "bar"
  
  str1.copy_somehow_another_builder(str2) #=&gt; "foobar"
  </code></pre>
  
  <p>currently I just <code>str1 &lt;&lt; str2.to_s</code>.</p>
  
  <p>How to do it?
  And is to_s'ing and pushing is same as what I want from performance point of view?</p>