Proposal: merge Data.Functor.Coproduct into transformers

[Ross Paterson]

On Sat, Dec 15, 2012 at 09:24:48PM +0000, Henning Thielemann wrote:
> Since I proposed that and became aware of the newtype solution in the 
> meantime, I change my mind to:
> 
> > data Sum f g a = Sum { getSum :: Either (f a) (g a) }

OK, let's do that.  It matches the treatment of Product.
(So now they'll both clash with Data.Monoid.)