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.)
* Markdown writer now includes a blank line at the end
of the row in a single-row multiline table, to prevent it from being
interpreted as a simple table. Closes#4578.
* Markdown reader does a better job computing the relative width of
the last column in a multiline table, so we can round-trip tables
without constantly shrinking the last column.
Currently the support for the `.. table` directive is a bit
limited; we don't yet support the `widths` field. But at least
you can have a proper captioned table.
