Create a specific decoda parser only if required

<table>
  <tr>
    <th>Q</th><th>A</th>
  </tr>
  <tr>
    <td>Bug fix?</td><td>no</td>
  </tr>
  <tr>
    <td>New feature?</td><td>no</td>
  </tr>
  <tr>
    <td>BC breaks?</td><td>no</td>
  </tr>
  <tr>
    <td>Tests pass?</td><td>yes</td>
  </tr>
  <tr>
    <td>License?</td><td>MIT</td>
  </tr>
</table>