Proposal: Add Foldable helper to Control.DeepSeq

[David Feuer]

As I describe in https://github.com/haskell/deepseq/issues/17 it is
possible to implement an NFData instance for any Foldable type, and we
can offer a function to help do that:

data Unit = Unit

instance Monoid Unit where
  mempty = Unit
  Unit `mappend` Unit = Unit -- strict in both arguments, unlike ()

rnfFoldable :: (Foldable f, NFData a) => f a -> ()
rnfFoldable xs = foldMap (\x -> rnf x `seq` Unit) xs `seq` ()

This could be used like so:

instance NFData a => NFData (F a) where
  rnf = rnfFoldable

This version forces from left to right. It would be possible to offer
another version that forces from right to left:

data Unit2 = Unit2

instance Monoid Unit2 where
  mempty = Unit2
  x `mappend` Unit2 = x `seq` Unit2

rnfFoldableRTL :: (Foldable f, NFData a) => f a -> ()
rnfFoldableRTL xs = foldMap (\x -> rnf x `seq` Unit2) xs `seq` ()