refactor: add parser state metamethods and simplify.

* lib/zile/syntax.lua (push_op): Moved from here...
(metatable.push_op): ...to here.  Adjust all callers.
(state.new): Set the metatable on the new parser state.
(highlight): Cache parser attrs and ops in locals for faster
access.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>