repurpose RValue as RegisterSlot

with the idea of the better name came also the one about not needing the builder anymore

test/risc/test_register_value.rb

@@ -54,7 +54,7 @@ module Risc
     end
     def test_index_op
       message = @r0[:next_message]
-      assert_equal RValue , message.class
+      assert_equal RegisterSlot , message.class
       assert_equal :next_message , message.index
       assert_equal @r0 , message.register
     end