a little refactoring (different CML loops have their own files now)

imperative-shell/update-thread.sml

@@ -0,0 +1,18 @@
+structure UpdateThread =
+struct
+  open CML
+
+  local
+    fun loop (inputMailbox, drawMailbox, model) =
+      let
+        val inputMsg = Mailbox.recv inputMailbox
+        val (model, drawMsg) = AppUpdate.update (model, inputMsg)
+        val _ = Mailbox.send (drawMailbox, drawMsg)
+      in
+        loop (inputMailbox, drawMailbox, model)
+      end
+  in
+    fun run (inputMailbox, drawMailbox, initial) =
+      loop (inputMailbox, drawMailbox, initial)
+  end
+end