Proposal: add "writer" Monad instance (,) o (Ticket #1951)

[Ross Paterson]

On Sun, Dec 02, 2007 at 09:27:06AM -0800, Conal Elliott wrote:
> I'd like to have a (,)-style writer instance alongside the (->)-based reader
> instance for Monad in Control.Monad.Instances.

Might as well, as there is already a matching Applicative instance.
Control.Monad.Instances is the right place to put the Monad instance, to
avoid breaking H98 compatibility.  (I'm not sure the Control.Applicative
dependency is worth it, but that's an internal detail.)