Add get_powers_of_2 function to spam_lists.structures
Add get_powers_of_2 function to spam_lists.structures

The function is basically SumClassificationCodeMap._get_codes method refactored
as a module-level function with a name that reflects the nature of operation
better and a clearer description in its docstring. Also, the return value is
changed from generator to a list.

piotr-rusin/spam-lists