Unclosed attributes disappear instead of turning into regular text

[hellux]

E.g.

text{a=x

{a=x
# heading

generates

<p>text</p>
<p></p>
<section id="heading">
<h1>heading</h1>
</section>

instead of

<p>text{a=x</p>
<p>{a=x
# heading</p>

Similar to #65

[jgm]

With this push we get

<p>text{a=x</p>
<p>{a=x</p>
<section id="heading">
<h1>heading</h1>
</section>

Still not quite right. If {a=x before # heading is just regular text, then we shouldn't get a heading at all.

We need to put some code into the block parser that is similar to what I just added to the inline parser. Better, make this backtracking code a method on the attribute parser.

[jgm]

Reverting for now because this commit caused fuzz test failures. Need to rethink.

[jgm]

Closed by c68e807cc3343364b2570c6f69a8eaeeb17d0ece and earlier commits.