XDOC support for KaTeX, and supporting updates

 - XDOC now supports LaTeX-like formulas within new @([...]) and @($...$)
   directives.  This is experimental.  See XDOC::KATEX-INTEGRATION for details.
   I would greatly appreciate any reports of web-browser incompatibilities
   related to this feature (or other xdoc features, for that matter).

 - OSLIB has been reorganized somewhat to try to make it more coherent.  Many
   files in oslib are now split up into, e.g., argv-logic.lisp (no raw code or
   ttags) and argv.lisp (actual implementation).

 - OSLIB has new functions for getting the dirname/basename from a path.
   These may be somewhat buggy since Lisp pathname stuff is awful.  See
   OSLIB::BASENAME and OSLIB::DIRNAME.

 - OSLIB has new functions for copying files and directories, See OSLIB::COPY.

 - Slightly extended Quicklisp directory with UIOP library and a more sensible
   CL-FAD book, in support of the above OSLIB changes.

 - XDOC::SAVE now uses OSLIB::COPY to copy the entire xdoc/fancy directory
   into the new manual.  (This avoids big ugly lists of files in the xdoc
   source code.)

 - Documented new #{"""..."""} string literal syntax, see
   ACL2::FANCY-STRING-READER.