Potential deadlock in `simHasFS`

[jorisdral]

fs-sim/fs-sim/src/System/FS/Sim/STM.hs

Lines 77 to 86 in 2d85d65

	sim :: FSSimT m a -> m a
	sim m = do
	st <- atomically $ takeTMVar var
	runFSSimT m st >>= \case
	Left e -> do
	atomically $ putTMVar var st
	throwIO e
	Right (a, st') -> do
	atomically $ putTMVar var st'
	pure a

If an exception happens between takeTMVar and putTMVar, then the TMVar will be left empty. This is a potential deadlock situation