Support generation of recursive data structures

[jcornaz]

One should be able easily and safely create a generator for a data-structure such as :

sealed class Tree {
  data class Branch(left: Tree, right: Tree) : Tree()
  object Leaf : Tree()
}

Maybe this is already possible with the current API. In which case the only thing to do is to add an example in the documentation.

If it is not already possible, then the necessary utilities should be provided.