Add voidM to Control.Monad

[Simon Hengel]

> >> voidM :: Monad m => m a -> m ()
> >> voidM m = m >> return ()
> >
> > Is voidM meant to be used with instance of Monad that are no monads, and
> > hence have no Functor instance (like HtmlM[1])?
> 
> You could easily implement a Functor instance for this type, since it
> would mirror the instance for `Const a` [2], i.e. only change the type
> parameter, and otherwise be the identity function.

Yes, you are right.  As long as we do not use bind we are fine.

Cheers,
Simon