Rebuild the Checkout class, no discounts yet

Checkout now stores its items using a quantity and a link back to the
original product instance. It has a reference to the store to make sure
that product IDs added are valid

product_in_checkout.rb

@@ -0,0 +1,22 @@
+class ProductInCheckout
+  def initialize(product:, quantity: 0)
+    @product = product
+    @quantity = quantity
+  end
+
+  def product_id
+    @product.id
+  end
+
+  def quantity
+    @quantity
+  end
+
+  def add_quantity(quantity)
+    @quantity += quantity
+  end
+
+  def total
+    @product.price * quantity
+  end
+end