Reduce symbol sizes and simplify iterator use

test/custom-formatter-test.cc

@@ -39,10 +39,9 @@ class custom_arg_formatter
 
 std::string custom_vformat(fmt::string_view format_str, fmt::format_args args) {
   fmt::memory_buffer buffer;
-  fmt::detail::buffer<char>& base = buffer;
   // Pass custom argument formatter as a template arg to vwrite.
-  fmt::vformat_to<custom_arg_formatter>(std::back_inserter(base), format_str,
-                                        args);
+  fmt::vformat_to<custom_arg_formatter>(
+    fmt::detail::buffer_appender<char>(buffer), format_str, args);
   return std::string(buffer.data(), buffer.size());
 }