Box helper to wrap types where WHNF = NF

I'd like a type that can be used to wrap values for which I want WHNF = NF. This is useful for satisfying NFData constraints with criterion for example.

```
newtype WHNF a = WHNF a
instance NFData (WHNF a) where rnf (WHNF a) = seq a ()
```

Of course, this data type should be given a better name. Maybe `data` instead of `newtype` would also be useful for even more laziness?
