Represent ItemRefs as an ADT

[lippo97]

Like this:

  sealed trait Item
  sealed trait ItemRef
  final case class SimpleItem(name: String)                         extends ItemRef
  final case class DecoratedItem(decoration: String, item: ItemRef) extends ItemRef
  val itemRetriever: Map[ItemRef, Item] = ???

  val a = SimpleItem("key")
  val b = DecoratedItem("red", SimpleItem("key"))
  val c = DecoratedItem("big", SimpleItem("key"))
  DecoratedItem("big", DecoratedItem("big", SimpleItem("key")))