Previously we used our own homespun formatting. But this
produces over-long lines that aren't ideal for diffs in tests.
Easier to use something off-the-shelf and standard.
Closes#7580.
Performance is slower by about a factor of 10, but this isn't
really a problem because native isn't suitable as a serialization
format. (For serialization you should use json, because the reader
is so much faster than native.)
Background: syntactically, references to example list items
can't be distinguished from citations; we only know which they
are after we've parsed the whole document (and this is resolved
in the `runF` stage).
This means that pandoc's calculation of `citationNoteNum`
can sometimes be wrong when there are example list references.
This commit partially addresses #6836, but only for the case
where the example list references refer to list items defined
previously in the document.
