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.
