[racket-dev] scriblib/figure

I've overhauled `scriblib/figure' to make it work better, render better
(e.g., with less content padding in HTML), and to be more configurable
via CSS/Latex.

The changes are not backward compatible for any use of
`scriblig/figure' combined with overriding CSS or Latex macros to
configure the output. The relevant style names were formerly
undocumented, anyway, and now they're documented.

Also, the output uses a slightly different pile of macros and
environments under Latex. Since the interaction of different Latex
environments is difficult to predict, it's possible that the changes
will break details of an existing document's rendering; let me know if
that happens.