Add mg_json_next() for iterating objects/arrays

src/json.h

@@ -19,3 +19,5 @@ char *mg_json_get_hex(struct mg_str json, const char *path, int *len);
 char *mg_json_get_b64(struct mg_str json, const char *path, int *len);
 
 bool mg_json_unescape(struct mg_str str, char *buf, size_t len);
+size_t mg_json_next(struct mg_str obj, size_t ofs, struct mg_str *key,
+                    struct mg_str *val);