Rename SlotMaker to Variable

Feels like now with better names, i can start to work.

lib/slot_language/assignment.rb

@@ -2,20 +2,20 @@ module SlotLanguage
   # A Assignment makes SlotLoad. That means it stores the information
   # to be able to create a SlotLoad
   #
-  # Just like the SlotLoad stores two Slots, here we store two SlotMakers
+  # Just like the SlotLoad stores two Slots, here we store two Variables
   #
   class Assignment
-    # The two SlotMakers that become Slots in to_slot
+    # The two Variables that become Slots in to_slot
     attr_reader :left , :right
 
     def initialize(left , right)
       @left = left
       @right = right
-      raise "No Slot #{left}" unless left.is_a?(SlotMaker)
-      raise "No Slot #{right}" unless right.is_a?(SlotMaker)
+      raise "No Slot #{left}" unless left.is_a?(Variable)
+      raise "No Slot #{right}" unless right.is_a?(Variable)
     end
 
-    # create the SlotLoad, by creating the two Slots from the SlotMakers
+    # create the SlotLoad, by creating the two Slots from the Variables
     def to_slot(compiler)
       left_d = @left.to_slot(compiler)
       right_d = @right.to_slot(compiler)