move slot out of instruction dir

since it is not an instruction, just a helper
also doing this before rewriting slot recursively

lib/slot_machine/slot.rb

@@ -0,0 +1,52 @@
+module SlotMachine
+  # A Slot defines a slot. A bit like a variable name but for objects.
+  #
+  # PS: for the interested: A "development" of Smalltalk was the
+  #     prototype based language (read: JavaScript equivalent)
+  #     called Self https://en.wikipedia.org/wiki/Self_(programming_language)
+  #
+  # Slots are the instance names of objects. But since the language is dynamic
+  # what is it that we can say about instance names at runtime?
+  # Start with a known object like the Message (in register one), we know all it's
+  # variables. But there is a Message in there, and for that we know the instances
+  # too. And off course for _all_ objects we know where the type is.
+  #
+  # The definiion is an array of symbols that we can resolve to SlotLoad
+  # Instructions. Or in the case of constants to ConstantLoad
+  #
+  class Slot
+    # get the right definition, depending on the object
+    def self.for(object , slots)
+      case object
+      when :message
+        MessageSlot.new(slots)
+      when Constant
+        ConstantSlot.new(object , slots)
+      when Parfait::Object , Risc::Label
+        ObjectSlot.new(object , slots)
+      else
+        raise "not supported type #{object}"
+      end
+    end
+
+    attr_reader :known_object , :slots
+    # is an array of symbols, that specifies the first the object, and then the Slot.
+    # The first element is either a known type name (Capitalized symbol of the class name) ,
+    # or the symbol :message
+    # And subsequent symbols must be instance variables on the previous type.
+    # Examples:  [:message , :receiver] or [:Space , :next_message]
+    def initialize( object , slots)
+      raise "No slots #{object}" unless slots
+      slots = [slots] unless slots.is_a?(Array)
+      @known_object , @slots = object , slots
+      raise "Not known #{slots}" unless object
+    end
+
+    def to_s
+      names = [known_name] + @slots
+      "[#{names.join(', ')}]"
+    end
+
+
+  end
+end