change in register_names protocol

move to returning the attribute names
getting and setting can then be automated in the base class

lib/risc/instructions/load_constant.rb

@@ -15,8 +15,8 @@ module Risc
     attr_accessor :register , :constant
 
     # return an array of names of registers that is used by the instruction
-    def register_names
-      [register.symbol]
+    def register_attributes
+      [:register]
     end
 
     def to_s