Type class instances for isomorphic types

Related Gitter conversation: https://gitter.im/typelevel/cats?at=58f5f7e801d449152eb5c5dc

It may be nice to provide an easy way to create type class instances that simply delegate to the type class instances of an isomorphic type. For example, if I create final case class DecodeJson[A](decode: Kleisli[Either[String, ?], Json, A]), then I can easily create an iso functor (a bidirectional tranformation via a pair of DecodeJson ~> Kleisli[Either[String, ?], Json, ?] and Kleisli[Either[String, ?], Json, ?] ~> DecodeJson). It seems like I should be able to pass in this iso functor to create a Monad[DecodeJson] that simply delegates through to the Kleisli Monad and runs the transformations on both ends.

I mentioned this use-case as motivation for #992, but that ticket seems to be stalled, and I'm not sure whether we'll go through with it. I think that this functionality could be useful whether or not anything happens with #992.

Start with the linked Gitter conversation and related issue #992 to understand the proposed isomorphism API and unresolved design questions. Define what a delegated type class instance should expose and how the bidirectional transformations are supplied; done means a workable design with coverage for the DecodeJson/Kleisli example.