add the ability to have optional/"add-on" functionality for a data structure

[bshlgrs]

It would be nice to be able to just have a StackReductionMemoizer which knows how to act like a Deque like this:

ds StackReductionMemoizer[f] {
  insertAtEnd! <- 1
  reduce[f] <-1
  QueueMemoizer {
     insertAtFront <- 1
  }
}

and when you use the more complicated inner structure, the interface clearly shows that it's being used.