Makes slots linked list

slots used to ba an array of symbols
Now we have an object for each slot, that holds the name and the next_slot
relatively easy change, though quite broad

lib/sol/assignment.rb

@@ -33,7 +33,7 @@ module Sol
     # Derived classes do not implement to_slot, only slot_position
     def to_slot(compiler)
       to = SlotMachine::Slotted.for(:message , self.slot_position(compiler))
-      from = @value.to_slot_definition(compiler)
+      from = @value.to_slotted(compiler)
       assign = SlotMachine::SlotLoad.new(self,to,from)
       return assign unless @value.is_a?(CallStatement)
       @value.to_slot(compiler) << assign