Proposal: Add common instances for Control.Applicative.Const

[Ross Paterson]

On Mon, Oct 14, 2013 at 09:33:38AM -0700, Eric Mertens wrote:
> It would be handy for Control.Applicative.Const to have the various derivable
> instances (Eq, Ord, Read, Show).
> 
> In particular Read and Show would make debugging easier.

I'd suggest redefining getConst as a function rather than a field to
make the derived Read and Show instances simpler.  Ditto WrappedMonad.