Implement Tree::reduce function

[jenetics]

The Program::eval method in the io.jenetics.prog module allows to evaluate operations trees, Tree<? extends Op<T>, ?>. This concept can be generalized by a folding function in the Tree interface. The signature of the fold method could look like this:

interface Folder<U, T> {
    U fold(final T root, final List<U> children);
}
interface Tree<V, T extends Tree<V, T>> {
    <U> U reduce(final Folder<U, V> folder);
}

[jenetics]

Merged into r7.1.0.