In Fix.DataFlow, change the bag of pending nodes from LIFO to FIFO.

Tests have shown that failing to do this could slow down the computation
of Menhir's invariant by 15x. (Fix.DataFlow is not yet used by Invariant,
so this change makes little difference at the moment.)

fix/src/DataFlow.ml

@@ -52,8 +52,8 @@ module Run
      [unit] should suffice, but our minimal map API does not offer a [remove]
      function. Thus, we have to use a map of variables to Booleans.) *)
 
-  let pending : variable Stack.t =
-    Stack.create()
+  let pending : variable Queue.t =
+    Queue.create()
 
   let dirty : bool M.t =
     M.create()
@@ -64,7 +64,7 @@ module Run
   let schedule (x : variable) =
     if not (is_dirty x) then begin
       M.add x true dirty;
-      Stack.push x pending
+      Queue.push x pending
     end
 
   (* [update x' p'] ensures that the property associated with the variable [x']
@@ -109,11 +109,11 @@ module Run
   let () =
     try
       while true do
-        let x = Stack.pop pending in
+        let x = Queue.pop pending in
         M.add x false dirty;
         examine x
       done
-    with Stack.Empty ->
+    with Queue.Empty ->
       ()
 
   (* Expose the solution. *)