message: add in old unsigned char because carlo will provide that fix
diff: @@ -356,12 +356,13 @@ using string_view = basic_string_view<char>;
  using wstring_view = basic_string_view<wchar_t>;
  // A UTF-8 code unit type.
  -typedef char fmt_char8_t;
  +typedef unsigned char fmt_char8_t;
  /** Specifies if ``T`` is a character type. Can be specialized by users. */
  template <typename T> struct is_char : std::false_type {};
  template <> struct is_char<char> : std::true_type {};
  template <> struct is_char<wchar_t> : std::true_type {};
  +template <> struct is_char<fmt_char8_t> : std::true_type {};
  template <> struct is_char<char16_t> : std::true_type {};
  template <> struct is_char<char32_t> : std::true_type {};
  