A shop inventory and checkout

A small point-of-sale system. Load a stock list, print it with your own Display impls, sort and total it with derived traits, report bad lines with a real error type, check out orders, and apply discounts through a pricing trait.

Milestones

1. Items that print themselves
2. Sorted stock and category totals
3. Bad lines and a real error type
4. Checkout
5. Discounts with a trait
6. Polish: promotions in one place, and restock warnings