fix bugs related to parsing and loading file (did not implement new BNF grammar properly)

fcore/common-update.sml

@@ -48,12 +48,12 @@ struct
 
   fun getSaveSquaresMsg (model: app_type) =
     let
-    (* todo: reimplement, saving each layer to a different line 
-    val {canvasWidth, canvasHeight, squares, ...} = model
-    val str = CollisionTree.toSaveString (squares, canvasWidth, canvasHeight)
-    val msg = SAVE_SQUARES str
-    *)
-    in raise Fail "todo common-update.sml: reimplement saving"
+      val {layerTree, canvasWidth, canvasHeight, ...} = model
+      val str =
+        CollisionTree.toSaveString (layerTree, canvasWidth, canvasHeight)
+      val msg = SAVE_SQUARES str
+    in
+      (model, [FILE msg])
     end
 
   fun getLoadSquaresMsg model =