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/position/position.rb

@@ -67,13 +67,20 @@ module Risc
 
     def set(int)
       return int if int == self.at
+      trigger_changing( int )
       Position.set_to(self , int)
       @at = int
       trigger_changed
       int
     end
 
-    # helper to fire the event that the position changed
+    # helper to fire the event that the position is about to change
+    # the argument is the new position (as int)
+    def trigger_changing( to )
+      event_table[:position_changed].each { |handler| handler.position_changing( self , to) }
+    end
+
+    # helper to fire the event that the position has changed
     # Note: set checks if the position actually has changed, before fireing
     #     but during insert it is helpful to trigger just to set the next
     def trigger_changed
@@ -116,8 +123,9 @@ module Risc
 
     def next_slot
       return -1 if at < 0
-      self.log.debug "Next Slot @#{at.to_s(16)} for #{object.class} == #{(at + object.byte_length).to_s(16)}"
-      at + object.byte_length
+      slot = at + object.padded_length
+      self.log.debug "Next Slot @#{at.to_s(16)} for #{object.class}(#{object.padded_length.to_s(16)}) == #{(slot).to_s(16)}"
+      slot
     end
 
     ## class level forward and reverse cache