add position_chaning to event interface

by reacting to the change _before it happens, we can move any BinaryCode out of the way

So when Instruction are inserted and code gets inserted, we don't need to set up the correct listener explicitly (which is tricky across mathods and changing chains), but instead just move anything that is in the way along

lib/arm/constants.rb

@@ -130,6 +130,6 @@ module Arm
    def byte_length
      4
    end
-
+   alias :padded_length :byte_length
   end
 end