Improve and simplify the layout for datatypes

[Drup]

This implements #614 : It uses lists instead of tables to layout datatypes, and add some heuristics to simplify the layout when the documentation payload is a single paragraph.

In practice, the layout for a sum datatype with a single-paragraph comment now looks like this:

<ol>
  <li id="type-variant.TagA" class="anchored">
    <a href="#type-variant.TagA" class="anchor"></a>
    <code><span>| </span><span><span class="constructor">TagA</span></span></code>
    <span class="def-doc">
      <span class="comment-delim">(*</span>This comment is for <code>TagA</code>.<span class="comment-delim">*)</span>
    </span>
  </li>
  ......
</ol>

The span def-doc is changed for a div for longer documentation comments.
The CSS has not been adapted yet (cue @dbuenzli !).

[jonludlam]

we're waiting on the CSS for this, right?

[leviroth]

With the merge of #830, can this be closed?

[panglesd]

Yes!