Counting food chains

You will write the functions that count the food chains of a web and that find the animals that nothing eats.

The idea. A web is stored as a dictionary. For each eater it lists the foods. The food chains run from the producer to an animal that nothing eats.

Your tasks.

1. Finish top_animals. It returns the sorted list of the species that nothing eats.
2. Finish count_chains. It returns the number of food chains that start at the producer.