io-sim: label Async's TVar

The monadic action of Async is baked by a TVar.  Naming it helps
debuging deadlocks.

Author: coot

io-sim/src/Control/Monad/IOSim/Internal.hs

@@ -428,6 +428,7 @@ instance MonadAsync (IOSim s) where
     var <- newEmptyTMVarIO
     tid <- mask $ \restore ->
              forkIO $ try (restore action) >>= atomically . putTMVar var
+    labelTMVarIO var ("async-" ++ show tid)
     return (Async tid (readTMVar var))
 
   asyncThreadId _proxy (Async tid _) = tid