Proposal #3537: Add missing NFData instances to parallel

[Roel van Dijk]

> what's wrong with
>
> instance NFData (Data.IORef.IORef a) where
>    rnf r = r `seq` ()

Ah, you are right. I was being to clever for my own good :-) Perhaps
it works for all types where I used the "rnf x = rnf (x == x)"
trick. Should I attach a new patch to the ticket with those
definitions changed?