Deprecate Foldable for Either

[Francesco Ariis]

On Thu, Mar 02, 2017 at 05:59:18PM +0100, Andreas Abel wrote:
> Ok, Foldable is a formal condition for Traversable, but not actually used in
> the implementation of Traversable Either.  This still leaves room to
> implement Foldable for Either by
> 
>   instance Foldable (Either a) where
>     foldMap _ _ = error "Folding Either?  Naah, I don't think this is a good
> idea."

I don't fancy Foldable for Either either, but runtime failures are
ugly too.