message: Add reserved definition to ico sketch
diff: //! - [Wikipedia](https://en.wikipedia.org/wiki/ICO_(file_format))
  //! - [Wikidata](https://www.wikidata.org/wiki/Q729366)
  +let reserved : fun (f : Format) -> Repr f -> Format =
  + fun f => fun default =>
  + let _ <- f;
  + return default;
  +
  let image = {
  width <- u8 if 0 then 256,
  height <- u8 if 0 then 256,
  