Fix performance regression introduced with the parsing callback feature.

src/json.hpp.re2c

@@ -3964,7 +3964,7 @@ class basic_json
                         auto value = parse_internal(keep);
                         if (keep and keep_tag and not value.is_discarded())
                         {
-                            result[key] = value;
+                            result[key] = std::move(value);
                         }
                     }
                     while (last_token == lexer::token_type::value_separator);
@@ -4015,7 +4015,7 @@ class basic_json
                         auto value = parse_internal(keep);
                         if (keep and not value.is_discarded())
                         {
-                            result.push_back(value);
+                            result.push_back(std::move(value));
                         }
                     }
                     while (last_token == lexer::token_type::value_separator);