Add Distributive typeclass

[coltfred]

There is a typeclass call Distributive that is the dual of Traverse

trait Distributive[F[_]] extends Functor[F] {
  def distributel[G[_]:Functor,A,B](fa: G[A])(f: A => F[B]): F[G[B]]
}

The feature that's really handy with this encoding is that it's stack safe to distribute over a Functor of arbitrary "size". For instance:

scala> val f = {x:Int => x +1}
f: Int => Int = $$Lambda$1271/1964149457@6dbabd33

scala> val funcs = List.fill(10000)(f)
funcs: List[Int => Int] = List($$Lambda$1271/1964149457@6dbabd33, $$Lambda$1271/1964149457@6dbabd33, $$Lamb...)

scala> funcs.distribute(identity).apply(1)
res15: List[Int] = List(2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2 ...)

scala> funcs.sequence.apply(1)  //Stack overflow

Would this typeclass be welcome in cats?

[coltfred]

Haskell has it here

Scalaz has it here

[johnynek]

I would welcome it!

Anything lawful that has several instances I think are great.