Offer Foldable helper

It's possible to define a reasonable `NFData` instance for any `Foldable` type. We can offer a suitable `rnf` to help.

``` haskell
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` ()
```
