Add flipped fmap

[wren ng thornton]

Felipe Lessa wrote:
>    <$$> will be as concise and useful as <$>, maybe even more,
>    because `flip fmap` isn't valid.
> 
> 2) Simmetry, as the ticket says.

If we want to keep the analogies complete then we'll need to add 
($$)=flip($) as well. As a prefix function it has a perspicuous type 
(type raising (Smullyan's thrush), aka CPS translation) and I've used it 
in a number of projects because of that.

All the same, I would not consider it worthwhile to add to any library 
(excepting one which contains Data.Smullyan :)  And it only opens the 
way for defining (!$$), and then why not add (<!$$>), (<!**>),... Every 
combinator has its place, but often that place is in a local module so 
that users/maintainers can easily figure out what your line noise means.

-- 
Live well,
~wren