Method format_handler

extlibs/fmt/include/fmt/format.h:3146–3149

  using range = typename ArgFormatter::range;

  format_handler(range r, basic_string_view<Char> str,
                 basic_format_args<Context> format_args,
                 internal::locale_ref loc)
      : parse_context(str), context(r.begin(), format_args, loc) {}

  void on_text(const Char* begin, const Char* end) {
    auto size = internal::to_unsigned(end - begin);