Refactor slot rendering to use Fragment

[stanleyxu2005]

Slot render creates 2 div to embed actual content node

<td>
  <div>    <!-- elements[key] -->
    <div>   <!-- h('div', ...) -->
       Cell content
    </div>
  </div>
</td> 

This PR will remove the inner div created by "h('div',"

<td>
  <div>    <!-- elements[key] -->
     Cell content
  </div>
</td> 

Note that comparing with pure js implementation, named slot approach will embed context into nested div elements. My ultermate target is to avoid any nested div.