handle implicitly closed elements

[Rich-Harris]

Starting to dig into #258 and #224 and related issues. As a starting point, Svelte needs to do a better job of handling situations like these:

<!-- the <p> is never closed, but that's totally fine in HTML -->
<div><p>some text</div>

<!-- these <li> elements don't need closing tags -->
<ul>
  <li>a
  <li>b
  <li>c
</ul>

This PR addresses both cases, by noting which elements are forbidden from containing which other elements, and by closing all currently open elements until the one that corresponds to a particular closing tag