Add Applicative instance for STM

Hi, I was rewriting my threads package to use TVars instead of MVars when I noticed that the following gave a type error: new ∷ IO ThreadGroup new = atomically $ ThreadGroup <$> newTVar 0 <*> newTMVar () because of a missing Applicative instance for STM. So I propose adding one: http://hackage.haskell.org/trac/ghc/ticket/4076 Discussion period: a few days, since I think this is hardly controversial. Regards, Bas