title: How to convert Swift Bool? -> String?
body: <p>Given a <code>Bool?</code>, I'd like to be able to do this:</p>
  
  <pre><code>let a = BoolToString(optbool) ?? "&lt;None&gt;"
  </code></pre>
  
  <p>which would either give me <code>"true"</code>, <code>"false"</code>, or <code>"&lt;None&gt;"</code>.</p>
  
  <p>Is there a built-in for <code>BoolToString</code>?</p>