This improves on the original fix to #5285 by preventing
other mixed lists (lists with a mix of Plain and Para
elements) that were allowed given the original fix.
Inline math in `\(..\)`, display math in `\[..\]`, tex is now used.
Previously we'd "fake it with unicode" and fall back to tex when
that didn't work. But as of
3f50b95532
haddock supports latex math.
This is now the default for pandoc's Markdown.
It allows whitespace between the two parts of a
reference link: e.g.
[a] [b]
[b]: url
This is now forbidden by default.
Closes#2602.
