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/risc/machine.rb

@@ -117,16 +117,14 @@ module Risc
     #
     # start at code_start.
     def position_code(code_start)
-      prev_code = nil
       Parfait.object_space.types.values.each do |type|
         next unless type.methods
         type.methods.each_method do |method|
+          Position.log.debug "Position starts for method #{method.name}"
           last_code = CodeListener.init(method.binary)
           last_code.set(code_start)
           first_position = InstructionListener.init(method.cpu_instructions, method.binary)
           first_position.set( code_start + Parfait::BinaryCode.byte_offset)
-          last_code.position_listener( prev_code.object) if prev_code
-          prev_code = last_code
           code_start = last_code.next_slot
         end
       end