Add Traversable isOrdered() indicator and Ordered interface

[danieldietrich]

This comes handy when converting a specific Traversable to an ordered collection like SortedSet or PriorityQueue. We could get rid of using instanceof.

// old code
if (this instanceof SortedSet) {
    Comparator<T> comparator = ((SortedSet<T>) this).comparator();
    ...
} else if (this instanceof PriorityQueue) {
    Comparator<T> comparator = ((PriorityQueue <T>) this).comparator();
    ...
} else if (this instanceof ...) {
    Comparator<T> comparator = ((...<T>) this).comparator();
    ...
} else {
    ...
}

// new code
if (isOrdered()) {
    Comparator<T> comparator = ((Ordered<T>) this).comparator();
    ...
} else {
   ...
}

The new Ordered interface looks like this:

interface Ordered<T> {
    Comparator<T> comparator();
}

It will be implemented by SortedSet, PriorityQueue, ... The existing comparator() methods need an @Override annotation then.

[danieldietrich]

This issue goes adhere with #1488

[danieldietrich]

See also #1635

[ruslansennov]

Closed by #1711