Proposal: merge Data.Functor.Coproduct into transformers

[Mario Blažević]

	So far we have a consensus on the proposal (yay!), but no firm 
agreement on the naming. Of the two alternatives I've suggested, Sum 
appears to have edged ahead of Coproduct. Let me know if I 
misrepresented anybody's position:

Edward  Coproduct  (LeftF | RightF)
Erik    Sum
Wren    Coproduct  (InL | InR)
Gábor   ?          (LeftF | RightF)
Andreas Either1    (Left1 | Right1)
Sean    Sum        (InL | InR)
Henning Sum        no prefixes


	Unfortunately, there seems to be a preference for a new data type 
instead of a newtype wrapping for Either, which gives us three bike 
sheds to paint instead of one. I'm going to list some of the suggested 
alternatives here:

 > data Coproduct f g a = LeftF (f a) | RightF (g a)
 > data Sum f g a       = InL   (f a) | InR    (g a)
 > data Either1 f g a   = Left1 (f a) | Right1 (g a)
 > data Either  f g a   = Left  (f a) | Right  (g a) -- use qualification


	I like the Either1 naming, but only if it's a start of a new trend. In 
other words, that's a good choice only if it gains a consensus so that 
other higher-order types switch to the scheme in the future. If that 
doesn't happen, my second choice would be Sum (InL, InR).