Add `rwhnf` helper for NF=WHNF instances

Here's an idea for a helper I wanted to document:

``` haskell
-- | Reduce to weak head normal form
--
-- Useful for defining 'NFData' for types for which NF=WHNF holds.
--
-- > data T = C1 | C2 | C3
-- > instance NFData T where rnf = rwhnf
rwhnf :: a -> ()
rwhnf !_ = ()
```
