refactor/checkout (#8)

Co-authored-by: matst80 <mats.tornberg@gmail.com>
Reviewed-on: #8
Co-authored-by: Mats Törnberg <mats@tornberg.me>
Co-committed-by: Mats Törnberg <mats@tornberg.me>

pkg/checkout/mutation_inventory_reserved.go

@@ -0,0 +1,16 @@
+package checkout
+
+import (
+	"fmt"
+
+	messages "git.k6n.net/go-cart-actor/proto/checkout"
+)
+
+func HandleInventoryReserved(g *CheckoutGrain, m *messages.InventoryReserved) error {
+	if m == nil {
+		return fmt.Errorf("HandleInventoryReserved: nil payload")
+	}
+
+	g.InventoryReserved = m.Status == "success"
+	return nil
+}