Add recursive Tree factory methods

[danieldietrich]

I described it in the blog-post Object-Functional Concurrent Programming with Javaslang.

Signatures:

Node<T> of(T seed,  Function<? super T, Iterable<? extends T>> descend);

Future<Node<T>> ofAsync(T seed,  Function<? super T, Future<? extends Iterable<? extends T>>> descend);

Depends on: #1529

Example impl:

static <T> Future<Node<T>> ofAsync(T seed,  
        Function<? super T, Future<? extends Iterable<? extends T>>>
        descend) {
    return Future.of(promise -> descend.apply(seed)
        .onComplete(result -> result
            .onSuccess(children -> promise.completeWith(
                Future.sequence(
                    Iterator.ofAll(children).map(child -> ofAsync(child, descend))
                ).map(childNodes -> Tree.of(seed, childNodes))))
            .onFailure(promise::failure)));
}

[danieldietrich]

Maybe this should be closed. The module vavr-collectionx should not depend on vavr-concurrent.

[danieldietrich]

The sync case is fixed with #2121. We will not do the async case.