rework binary code positioning setup

dependency chain set up explicitly.
Next have to react to events correctly

lib/risc/position/code_listener.rb

@@ -72,6 +72,7 @@ module Risc
           at += code.padded_length unless at < 0
           code = code.next
         end
+        position
       end
     end
   end

lib/risc/position/object_position.rb

@@ -40,6 +40,10 @@ module Risc
         trigger(:position_changed , self)
         true
       end
+      def next_slot
+        return -1 if at < 0
+        at + object.byte_length
+      end
       def self.init(object , at = -1)
         position = ObjectPosition.new(object , at)
         Position.set_to( position , at)