Method on_fill

extlibs/fmt/include/fmt/format.h:2083–2085

  FMT_CONSTEXPR void on_align(align_t align) { specs_.align = align; }
  FMT_CONSTEXPR void on_fill(basic_string_view<Char> fill) {
    specs_.fill = fill;
  }
  FMT_CONSTEXPR void on_plus() { specs_.sign = sign::plus; }
  FMT_CONSTEXPR void on_minus() { specs_.sign = sign::minus; }
  FMT_CONSTEXPR void on_space() { specs_.sign = sign::space; }