to_s for reg instructions

lib/register/instructions/get_slot.rb

@@ -27,6 +27,11 @@ module Register
       raise "Not register #{array}" unless Register::RegisterReference.look_like_reg(array)
     end
     attr_accessor :register , :array , :index
+
+    def to_s
+      "GetSlot(#{register}: #{array}[#{index}])"
+    end
+
   end
 
   # Produce a GetSlot instruction.