Merge branch 'feature/parser_performance' into develop

- improved parser performance by avoiding a copy of the result value

src/json.hpp.re2c

@@ -7795,7 +7795,7 @@ class basic_json
 
             // return parser result and replace it with null in case the
             // top-level value was discarded by the callback function
-            return result.is_discarded() ? basic_json() : result;
+            return result.is_discarded() ? basic_json() : std::move(result);
         }
 
       private: