* 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.
In the reader, we use the new Table type in Haddock.
Note that tables with col/rowspans will not translate
well into Pandoc.
In the writer, we now render tables always as grid tables,
since Haddock supports these.
