Why is there no Foldable instance for Either?

[Ross Paterson]

On Wed, Jun 12, 2013 at 11:01:15AM -0400, Edward Kmett wrote:
> On Wed, Jun 12, 2013 at 5:10 AM, Shachaf Ben-Kiki <shachaf at gmail.com> wrote:
>     Just to make sure this isn't slipping through the cracks -- is there a
>     particular reason not to add the instance for (Const r)? It was
>     brought up in the last discussion and it has one obviously-correct
>     definition. It's a useful instance.
> 
> No reason at all other than an annoying major version bump in transformers.

Constant in transformers already has these instances, but Const in base
doesn't.  Otherwise they're the same (and both of them are my fault).
One of them should go -- I'm in favour of keeping the full name.