Make const versions of strings in structs to avoid string copying & pass objects to create/update instead of values

src/modb_metadata_ext.h

@@ -27,7 +27,10 @@ struct meta_ext_t {
 struct meta_ext_value_t {
   char *key;
   const char *key_c;
+  size_t key_len;
+
   enum e_value_type_t type;
+
   union {
     void *raw;
     int bool;