Explore alternatives to the state hack

The state hack (-fstate-hack, on by default, see Note [The state-transformer hack] in G.C.O.Arity) is a smart hack without which efficient compilation of IO wouldn't be possible. But fixes such as 69c0e16e made me thinking: Now that we have GHC.Magic.oneShot, can we get by without?

Suppose we define (in ghc-prim:GHC.Types):

newtype IO a = IONoEta (State# RealWorld -> (# State# RealWorld, a #))

pattern IO m <- IONoEta m
  where
    IO m = IONoEta (oneShot m)

Similarly for ST. I hope that we can get rid of the state hack this way. That would also fix #14596.

Another advantage to this is that we can use IONoEta on a use by use basis, rather than having the rather unobtrusive {-# OPTIONS_GHC -fno-state-hack #-} at the top of the module, disconnected from its usage.