Generalize pre-allocation of pairs

[dschrempf]

I am creating this issue so this TODO item is tracked on GitHub.

This is about preallocation of pairs:

-- | Preallocate two values
--
-- TODO: It would be nice to generalize this, but I can't quite figure out how
-- without introducing a ton of machinery.
preallocatePair :: forall a b r.
     (Preallocate a, Preallocate b)
  => (Writing a -> Writing b -> IO r)
  -> IO ((a, b), r)
preallocatePair k = fmap reassoc $
    preallocate $ \wa ->
    preallocate $ \wb ->
      k wa wb
  where
    reassoc :: (a, (b, r)) -> ((a, b), r)
    reassoc (a, (b, r)) = ((a, b), r)