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

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:

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

Discussion Period: 2 weeks