1. The new methods we added to the Order class are pure, but not all 
   the existing ones are. Purify the remaining functions, where possible
   and test your work.

2. Although we haven't used them so far, comprehensions are common
   in functional languages.  Create new, pure versions of the get_filtered_info
   and get_order_by_id methods that use list comprehensions.

3. Write a new version of the set_order_expedited method that uses the
   Order.map method instead of iteration and assignment.  Use the recipe for
   the consume function from the itertools module.

    See https://docs.python.org/3/library/itertools.html#itertools-recipes
