Implement nested-cases

[sviperll]

Suppose that you have a class that represents HTTP-endpoints

Like this:

    interface URLVisitor<R> {
        R userIndex();
        R user(int id);
        R postIndex(int userID);
        R post(int postID);
    }

During the application evolution it can be desired to refactor above visitor into nested visitors, like this:

    interface URLVisitor<R> {
        R userIndex();
        UserURLVisitor<R> user(int id);
    }

    interface UserURLVisitor<R> {
        R user();
        R postIndex();
        R post(int id);
    }

We should probably allow such definitions for ADT4J and generate sensible factory and accept methods:

    URL url = URL.factory().user(123).post(1);