[Proposal] Add instance Monoid a => Monoid (Const a b)

[Ross Paterson]

On Sun, Sep 29, 2013 at 12:13:34PM -0400, Edward Kmett wrote:
> A decent argument in favor is there really isn't another viable instance for
> it. Being able to glue together folds with (<>) never occurred to me.

I agree with that.  The only issue is that that we have two functors
that are identical:

- Const exported by Control.Applicative in base
- Constant exported by Data.Functor.Constant in transformers

We should decide on a single name, and which package it should be in.