julia-mode.el: Further syntax/highlighting bugfixes

These four commits improve Emacs' understanding of Julia syntax.

I've taken the syntax table improvements by @Clemens-H in #8026 and extended them. These ensure that `forward-word` and `forward-symbol` work correctly for Julia code, and enable us to simplify our regexps to just say `(or word (syntax symbol))`.

(I've also just realised that me preparing this branch has probably spammed Clemens with notifications, sorry about that!)

Finally, I've fixed #8537 by re-implementing `regexp-opt` in an Emacs 23 friendly way.