core: deepcopy notification in System.Runtime.Notify

Make it impossible to change already emitted notifications via
`System.Runtime.GetNotifications`.

pkg/core/interop/runtime/util.go

@@ -47,7 +47,7 @@ func GetNotifications(ic *interop.Context, v *vm.VM) error {
 		ev := stackitem.NewArray([]stackitem.Item{
 			stackitem.NewByteArray(notifications[i].ScriptHash.BytesBE()),
 			stackitem.Make(notifications[i].Name),
-			notifications[i].Item,
+			stackitem.DeepCopy(notifications[i].Item).(*stackitem.Array),
 		})
 		arr.Append(ev)
 	}

pkg/core/interop_system.go

@@ -282,7 +282,7 @@ func runtimeNotify(ic *interop.Context, v *vm.VM) error {
 	ne := state.NotificationEvent{
 		ScriptHash: v.GetCurrentScriptHash(),
 		Name:       name,
-		Item:       stackitem.NewArray(args),
+		Item:       stackitem.DeepCopy(stackitem.NewArray(args)).(*stackitem.Array),
 	}
 	ic.Notifications = append(ic.Notifications, ne)
 	return nil