Proposal: merge Data.Functor.Coproduct into transformers

[Mario Blazevic]

At the instigation of Edward Kmett, I hereby propose to merge
Data.Functor.Coproduct module from comonad-transformers into
transformers.

The module defines the Coproduct data type, together with several
class instances. All non-base instances would have to be left out,
leaving the following instances to be merged in:

  instance (Functor f, Functor g) => Functor (Coproduct f g)
  instance (Foldable f, Foldable g) => Foldable (Coproduct f g)
  instance (Traversable f, Traversable g) => Traversable (Coproduct f g)

Unfortunately, the Applicative and Monad instances cannot be defined
on this type, though their duals can. In that sense, the existing
comonad-transformers package is a more fitting home for the type.
However, there are good reasons for the move:

- The dual type of Coproduct, namely Data.Functor.Product, is already
defined by transformers. It would make sense to keep the duals
together.
- Data.Functor.Compose and Data.Functor.Constant which also defined by
transformers are not monads either, so there is precedent for having
non-monads there.
- Most obviously, transformers is a haskell-platform package while
comonad-transformers seems unlikely to become one any time soon (as
much as I'd like that).
- The existing Comonad instance can be moved from comonad-transformers
to comonad; it would not be orphaned.

The patch against the darcs repository is attached.

An alternative naming for the module (and the type) would be
Data.Functor.Sum. This name would be shorter and less frightening to
newcomers, but it clashes with its namesake type from Data.Monoid. On
the other hand, so does Data.Functor.Product. I'm ambivalent on the
naming, but I'm throwing this out because I don't expect it to become
a bikeshed issue. If you have any name preference, state it together
with your vote; if the proposal succeeds we can use whichever name is
preferred.
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