Proposal: Add common instances for Control.Applicative.Const

[Ross Paterson]

On Mon, Oct 14, 2013 at 07:43:31PM -0400, Edward Kmett wrote:
> Alternately we could just hand-roll the instances so it doesn't change whether
> it gets imported if the user writes 
> 
> import Control.Applicative (Const(..))
> 
> but I agree with simplifying the presentation.

Hand-rolling is possible, but it's more transparent to use the derived
instances.