split cart and checkout and checkout vs payments

pkg/checkout/mutation_payment_event.go

@@ -0,0 +1,20 @@
+package checkout
+
+import (
+	messages "git.k6n.net/go-cart-actor/proto/checkout"
+)
+
+func HandlePaymentEvent(g *CheckoutGrain, m *messages.PaymentEvent) error {
+
+	payment, found := g.FindPayment(m.PaymentId)
+	if !found {
+		return ErrPaymentNotFound
+	}
+	metaBytes := m.Data.Value
+	payment.Events = append(payment.Events, &PaymentEvent{
+		Name:    m.Name,
+		Success: m.Success,
+		Data:    metaBytes,
+	})
+	return nil
+}