free: hoistCofree should be generalized

[Gurkenglas]

It currently has type

Functor f => (forall x. f x -> g x) -> Cofree f a -> Cofree g a

It could have type

Functor f => (f (Cofree g a) -> g (Cofree g a)) -> Cofree f a -> Cofree g a

This would allow me to write things like

import qualified Data.Foldable as F
braid :: Monad m => Cofree m a -> [m a]
braid = F.toList . hoistCofree (Identity . join) . return

[neongreen]

Made a pull request: ekmett/free#131

[fumieval]

I think we deliberately use (forall x . f x -> g x) to allow only "legal" transformations.

[neongreen]

Okay, makes sense to me. @Gurkenglas, can this be closed?

[neongreen]

(There's a longer explanation here: ekmett/free#131 (comment).)