Allow generic types in shared traits

[mariusmuja]

This PR adds support for higher kinded generic parameters to use for return types in the shared interface, such as:

// shared interface
trait Api[F[_]]{
  def add(x: Int, y: Int, z: Int): F[Int]
}

This is very useful when using tagless final encoding for the algebras used in the program.

For macro application, one needs to use Future for the generic type, for example:

Client[Api[Future]].add(1, 2, 3).call()

but it's possible to make use libraries such as mainecoon, to have the Future interpreter automatically deduced from any other type for which a natural transformation is available.