Creates a SplayTreeSet from elements.
The set works as if created by new SplayTreeSet<E>(compare, isValidKey).
All the elements should be valid as arguments to the compare function.
factory SplayTreeSet.of(Iterable<E> elements,
[int compare(E key1, E key2), bool isValidKey(potentialKey)]) =>
new SplayTreeSet(compare, isValidKey)..addAll(elements);