Method format_handler

include/fmt/format.h:3977–3979

            buffer_context<Char> context;

            format_handler(buffer_appender<Char> p_out, basic_string_view<Char> str,
                           basic_format_args<buffer_context<Char>> p_args, locale_ref p_loc)
                : parse_context(str), context(p_out, p_args, p_loc) {}

            void on_text(const Char* begin, const Char* end) {
                auto text = basic_string_view<Char>(begin, to_unsigned(end - begin));