Converts the products in a cart to the format Criteo expects for its `items` property

@param {Object} cart - cart as `CartValue` object
@returns {Array} - An array of items to include in the Criteo tracking call
function cartToCriteoItems( cart ) {
	return cart.products.map( product => {
		return {
			id: product.product_id,
			price: product.cost,
			quantity: product.volume,
		};
	} );
}