removing those ugly slot index constants

The constants were bad enough,
but they were also at the wrong level

Now register level is defining mappings from
symbol names to indexes, by using the layout

lib/virtual/slots/slot.rb

@@ -2,7 +2,8 @@ module Virtual
   # A slot is a slot in an object. It is the storage location for a value.
   #             (Remember, values are typed)
   # From a memory perspective a slot is an index into an array (the object)
-  # We are not modelling the array here, but the index into it.
+  # The mapping into arrays is a straightforward matter, but happens in the
+  # next level down, the register machine.
 
   # Four known objects exist and those correspond to subclasses:
   # - the message that has been received: MessageSlot
@@ -17,12 +18,11 @@ module Virtual
 
   class Slot < Object
 
-    attr_accessor :index , :type , :value
+    attr_accessor :type , :value
 
     private #abstract base class
 
-    def initialize index , type , value
-      @index = index
+    def initialize type , value
       @type = type
       @value = value
     end