Refactoring: move UTF-8 decoding functions into coding.h.

Refactoring: move UTF-8 decoding functions into coding.h.

json_make_string and json_build_string are generally useful and not
JSON-specific.  Move them to coding.[ch].

* src/coding.h (build_utf8_string): Move from json.c.

* src/coding.c (make_utf8_string): Move from json.c.

* src/json.c (json_make_string, json_build_string): Move to
coding.[ch].  Split out JSON-specific comment.
(json_parse_error, Fjson_serialize, json_to_lisp): Fix callers.

* src/emacs-module.c (module_make_function, module_make_string): Use
new functions.
(module_decode, module_decode_copy): Remove.