rename register to risc

seems to fit the layer much better as we really have a very reduced
instruction set

lib/risc/instructions/operator_instruction.rb

@@ -0,0 +1,21 @@
+module Risc
+
+  class OperatorInstruction < Instruction
+    def initialize source , operator , left , right
+      super(source)
+      @operator = operator
+      @left = left
+      @right = right
+    end
+    attr_reader :operator, :left , :right
+
+    def to_s
+      "OperatorInstruction: #{left} #{operator} #{right}"
+    end
+
+  end
+  def self.op source , operator , left , right
+    OperatorInstruction.new source , operator , left , right
+  end
+
+end