And similar for `\midrule` and `\bottomrule`.
This facilitates redefining `\toprule`, `\midrule`, and `\bottomrule`
without needing to gobble the ()s.
Closes#8223.
The writer uses features from the longtable package to define the table
foot. Furthermore, the table's bottom rule is now part of the foot,
where it previously was given as part of the body.
The caption of headless tables was repeated on each page that contained
part of the table. It is now made part of the "first head", i.e. the
table head that is printed only once.
John MacFarlane