Add flipped fmap

[Bas van Dijk]

Hello,

I would like to propose adding a flipped variant of <$> to Data.Functor:

(<$$>) ∷ Functor f ⇒ f α → (α → β) → f β
(<$$>) = flip (<$>)

Please see the ticket for motivation and patch:

http://hackage.haskell.org/trac/ghc/ticket/3962

Discussion period: 2 weeks.

regards,

Bas