CofreeEqual with Implicit Function Types?

cofreeEqual.scala

```scala
implicit def cofreeEqual[A, F[_]](implicit A: Equal[A], F: Equal[F[Cofree[F, A]]]): Equal[Cofree[F, A]] =
  Equal.equal{ (a, b) =>
    A.equal(a.head, b.head) && F.equal(a.tail, b.tail)
  }
  
scala> import scalaz._,Scalaz._
import scalaz._
import Scalaz._

scala> Cofree.cofreeEqual[Int, Option]
<console>:14: error: diverging implicit expansion for type scalaz.Equal[Option[scalaz.Cofree[Option,Int]]]
starting with method optionEqual in trait OptionInstances0
              Cofree.cofreeEqual[Int, Option]
                                ^
```

might be resolved by "implicit function types", something like:

```scala
type CofreeEqual[A, F[_]] = implicit Equal[A] => implicit Equal[F[Cofree[F, A]]] => Equal[Cofree[F, A]]
implicit def cofreeEqual[A, F[_]]: CofreeEqual[A,F[_]] =
  implicitly[Equal[A]].equal{ (a, b) =>
    A.equal(a.head, b.head) && implicitly[Equal[F[Cofree[F,A]]]].equal(a.tail, b.tail)
  }
```

referring to @xuwei_k 's blog post:
http://d.hatena.ne.jp/xuwei/20131223/1387798959