Proposal: Add "writer" Monad instance (, ) o to Control.Monad.Instances

[Jake McArthur]

This proposal [1] was originally submitted to Trac by Conal Elliott, but 
it was apparently abandoned and closed after some time. I've picked it 
back up and written out a patch. Here is the text of the proposal:

> I'd like to have a (,)-style writer instance alongside the (->)-based reader instance for Monad in Control.Monad.Instances.
>
> Here's the current reader:
>
>     instance Monad ((->) r) where
>         return = const
>         f >>= k = \ r -> k (f r) r
>
> and my proposed writer:
>
>     instance Monoid o => Monad ((,) o) where
>         return = pure
>         (o,a) >>= f = (o `mappend` o', a') where (o',a') = f a
>
> where the return definition relies on the Applicative instance of ((,) o). Written out explicitly,
>
>         return a = (mempty,a)
>
> Control.Monad.Instances will also need two new imports:
>
>     import Data.Monoid (Monoid(..))
>     import Control.Applicative (pure)

- Jake

[1] http://hackage.haskell.org/trac/ghc/ticket/1951