Add pseudo-instruction for reserved opcodes
Add instructions for DELAY/FORCE

Add the LDE, AP0, and UPD instructions from Chapter 8 of
Henderson (1980) -- with the following alterations:  we will not
distinguish between an unevaluated receipe and an evaluated recipe.
A recipe is always unevaluated.  An "evaluated recipe" is just a
value.  The implications of this are:

  - AP0 doesn't necessarily expect a recipe on the top of the stack.  If
    the item on the top of the stack is not a recipe, AP0 acts like NOP.
    If it is a recipe, it behaves as described in the book.
  - UPD replaces (recipe:(c.e)) with the result of the evaluation, not
    with an "evaluated recipe" that refers to the result of the
    evaluation.
Add support for DELAY/FORCE to LispKit
Print recipes as '?'