newTVar isn't properly handled in snapshots

[barrucadu]

With this test case:

example0 :: Program (WithSetup (ModelTVar IO Int)) IO Int
example0 = withSetup (atomically $ newTVar 0) $ \tvar -> do
  atomically $ modifyTVar tvar (+1)
  atomically $ readTVar tvar

We get the totally nonsensical:

> putStr . unlines . map (\(a,b) -> show a ++ "     " ++ showTrace b) =<< runSCT (randomly (mkStdGen 0) 10)  defaultMemType example0                                                                       
Right 1     S0---                                                                                                                                                                                          
Right 2     S0---                                                                                                                                                                                          
Right 3     S0---                                                                                                                                                                                          
Right 4     S0---                                                                                                                                                                                          
Right 5     S0---                                                                                    
Right 6     S0---                                                                                                                                                                                          
Right 7     S0---                                                                                                                                                                                          
Right 8     S0---                                                                                                                                                                                          
Right 9     S0---                                                                                                                                                                                          
Right 10     S0---

Oh no!

The issue is that snapshots don't restore the initial value of a TVar.

I think fixing this covers up the problem in #323, but I suspect there is still an underlying bug in #323 somewhere - even if the TVar value isn't being reset correctly, it shouldn't be changing!