Add flipped fmap

[Roel van Dijk]

+1 from me for inclusion of <$$>. Mainly because of the symmetry
argument. When you look at the types of (<*>), (<**>); (>>=), (=<<)
and (<$>) you can expect there to be a operator with (<$$>)'s type.
Its non-existence is surprising. I like the law-of-least-surprise in
library design.

I do not like the specific symbol <$$>. It is similar to <**> which is
nice, but I also don't like <**>. I prefer some sort of visual
indication of direction. Like (>>=) versus (=<<) or (⋙) versus (⋘).
But in this case history wins over aesthetics.

Roel