Move pure+side-effect-free declarations to byte-opt.el

Move pure+side-effect-free declarations to byte-opt.el

* src/json.c (syms_of_json): Move pure and side-effect-free declarations
for json-serialize and json-parse-string from here...
* lisp/emacs-lisp/byte-opt.el (side-effect-free-fns, pure-fns):
...to here.

(cherry picked from commit bef0d80be88906b66dec06970d7782f0f7b219c0)