move msgs into AppType.app_type, and refactor AppUpdate.update function to have type (app_type -> app_type) instead of pervious case which was (app_type -> app_type * MailboxType.t list). Reason: no need to allocate/return tuple when can store messages in type directly; previous msg list will be reset after each app update. All tests after refactor are passing.

shell/update-thread.sml

@@ -17,8 +17,8 @@ struct
   fun loop (app: AppType.app_type, inputMailbox, drawMailbox) =
     let
       val inputMsg = Mailbox.recv inputMailbox
-      val (app, msgList) = AppUpdate.update (app, inputMsg)
-      val _ = sendMsgs (msgList, drawMailbox)
+      val app = AppUpdate.update (app, inputMsg)
+      val () = sendMsgs (#msgs app, drawMailbox)
     in
       loop (app, inputMailbox, drawMailbox)
     end