Add Applicative instance for STM

[Bas van Dijk]

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