Exercise 6

Write a function add-ingredients which takes
two ingredient lists and adds them together using
merge-with.
Exercise 7

Write a function multiply-ingredients
that takes a quantity and an ingredient list and
returns a new ingredient list with all the
amounts multiplied by the quantity. Use into
and for.
Exercise 8

Write a function order->ingredients which
takes an order and returns an ingredient list for
everything needed in that order. You should
use add-ingredients and multiply-ingredients.
Exercise 9

Write a function orders->ingredients that
builds a total ingredients list from a list of
orders. You should use map and reduce.