Add hoistCofree

[exergasia]

Please forgive me if this is already defined somewhere--I couldn't find a precise match--but this function would be useful to have:

hoistCofree :: (Functor f) => (forall x . f x -> g x) -> Cofree f a -> Cofree g a
hoistCofree f (x :< y) = x :< f (hoistCofree f <$> y)

[ekmett]

Toss a patch together and i'll happily accept it.

[fumieval]

As it has no harm, I've just added it (a2008da).

[ekmett]

Fair enough. =)