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

[haskell at nand.wakku.to]

Hello,

I think this a Monoid instance on Const would be useful. The motivation comes
from ‘lens’, where folds come out as (a -> Const a a) -> s -> Const a s. This
instance, together with the Monoid a => Monoid (r -> a) instance, allows folds
themselves to be an instance of Monoid, allowing for some really pretty code.

Example of the instance body:

> instance Monoid a => Monoid (Const a b) where
>   mempty = Const mempty
>   mappend (Const a) (Const b) = Const (mappend a b)

Examples of it being used together with lens:

>>> "Hello, world!" ^.. mconcat [ix 2, ix 7, ix 1]
"lwe"

Discussion Period: 2 weeks