propagate instruction positions

still overlapping onto binaries, but a start

test/risc/position/helper.rb

@@ -5,8 +5,14 @@ module Risc
     def padded_length
       4
     end
+    def byte_length
+      4
+    end
   end
   class DummyInstruction < Dummy
     include Util::List
+    def initialize(nekst = nil)
+      @next = nekst
+    end
   end
 end