introduce the LabeListener to move instructions along when first code position changes

test/risc/position/test_label_listener.rb

@@ -0,0 +1,25 @@
+require_relative "helper"
+
+module Risc
+  class TestLabelListener < MiniTest::Test
+
+    def setup
+      Risc.machine.boot
+      @label = Label.new("Hi","Ho" , FakeAddress.new(5))
+      @label_pos = Position.new(@label , -1)
+      @code = Parfait::BinaryCode.new(1)
+      @code_pos = Position.new(@code , -1)
+      @code_pos.position_listener( LabelListener.new(@label))
+    end
+
+    def test_move
+      @code_pos.set(0)
+      assert_equal 8 ,  @label_pos.at
+    end
+    def test_move_twice
+      @code_pos.set(0)
+      @code_pos.set(0x40)
+      assert_equal 72 ,  @label_pos.at
+    end
+  end
+end