This is so we can strip out support code needed to make the examples work,
and as a bonus it does syntax highlighting.

And adjust highlighting/testing scripts to deal with this.

Refactor the doc building rules a bit, sensitize them to the presence or absence of xetex and luatex (for now).

Begin shift over to using pandoc, markdown and llnextgen for reference manual. Fix man page URL while at it.

Keywords are now listed in a plain text file. They're sorted in
column-major order and rendered as a texinfo multitable in rust.texi.

Fixes issue #1216.

Naturaldocs isn't really that great but it seems easier to get
something working than with doxygen, for which we would need to
convert rust code to something C++ish. We probably want to just
write a rustdoc utility at some point.

Also added the --batch flag to texi2pdf, as it doesn't really ever
seem useful to drop to the TeX prompt during a build.