Tiny JSON performance improvement

Tiny JSON performance improvement

Get rid of some needless uses of apply.  Measuring with
  (benchmark-run 10 (json-read-file "test.json"))
showed 1.5-2.5% reduction of execution time.
* lisp/json.el (json-peek): Nix let-binding.
(json-read-string): Use concat for making a string from chars.
(json-read-array): Use cond and more appropriate conversion instead
of blindly applying.