1. Using the solution from the previous module, change it to return      
iterators (as generators) instead of tuples whereever possible
2. Changet the set expedited orders function to use the new action_if function
3. Ensure the mark_backordered function so that it still returns a 
   tuple, since this will become the new master collection
4. (hint) Create a new function, mark_backordered_items to factor out
   the logic handling the order_items collection