More JSON optimization

More JSON optimization

Last I checked, inlining json-skip-whitespace didn't make much
difference.  However, changing defsubsts to define-inline results
in roughly 15% reduction in read time on a 200K file.
* lisp/json.el (json-advance, json-peek, json-pop):
(json-skip-whitespace): Inline with define-inline.
(json-read-keyword): Don't use whitespace syntax.
(json-add-to-object): Simpler condition.