STM actions should be undone in reverse order

[barrucadu]

This is kind of embarrassingly bad:

test :: MonadConc m => m Int
test = do
  v <- atomically (newTVar 1)
 fork . atomically $ do
    writeTVar v 2
    writeTVar v 3
    retry
  atomically (readTVar v)

Because the writing transaction is unconditionally aborted, this is equivalent to:

test :: MonadConc m => m Int
test = do
  v <- atomically (newTVar 1)
  fork (atomically retry)
  atomically (readTVar v)

But dejafu thinks it's nondeterministic:

> runST $ resultsSet defaultWay defaultMemType test
fromList [Right 1,Right 2]

The problem is here, in the implementation of rolling back transactions:

dejafu/dejafu/Test/DejaFu/STM/Internal.hs

Line 154 in 05a8f4a

newUndo = undo >> undo'

The >> should have its arguments flipped. Currently the undo action is computed as writeTVar v 1 >> writeTVar v 2, which is clearly wrong.