Shared.hierarchicalize: improve handling of div and section structure.

Previously Divs were opaque to hierarchicalize, so headings
inside divs didn't get into the table of contents, for
example (#3057).

Now hierarchicalize treats Divs as sections when appropriate.
For example, these structures both yield a section and a
subsection:

``` html
<div>
<h1>one</h1>
<div>
<h2>two</h2>
</div>
</div>
```
``` html
<div>
<h1>one</h1>
<div>
<h1>two</h1>
</div>
</div>
```

Note that

``` html
<h1>one</h1>
<div>
<h2>two</h2>
</div>
<h1>three</h1>
```

gets parsed as the structure

    one
      two
    three

which may not always be desirable.

Closes #3057.

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