Add method to perform natural transformation of underlying Monad in Monad Transformers

[LukaJCB]

I've found myself in this position quite often and I think it might be very useful to be able to change the underlying Monad of a Monad Transformer.
For example in OptionT:

final case class OptionT[F[_], A](value: F[Option[A]]) {
  ...
  def transformF(fn: F ~> G): OptionT[G, A] = OptionT(fn(value))
  ...
}

We could add this to all Monad Transformers currently in cats, i.e.

• OptionT
• EitherT
• StateT
• WriterT
• Kleisli

The name is debatable, other suggestions one might come up with are: mapF, mapK, or just transform.

Edit: Just realized Kleisli already has a method that does exactly this called transform so we should probably stick to that.

[LukaJCB]

There also seems to be #1644 which adds a transformF function that also does the same thing to EitherT. I suggest we find a common name for this function (with the precedent of transform already available in Kleisli)

Edit: I see now, that EitherT already has a function called transform, so we might need to find a different solution. This makes it very difficult to find a common name.

We could either duplicate the function with the common name in Kleisli or EitherT or choose to break backwards compatibility. Personally, right now, I think I'd go for the former

[kailuowang]

This might be calling a higher order Functor, mentioned in #1644

[kailuowang]

also related, @edmundnoble has a HFunctor like type class in cats-mtl
https://gitter.im/typelevel/cats-dev?at=593b10d86462d8493cf8d3c4

[edmundnoble]

This seems to be an exact duplicate of #1713.