Reconsider ImmutableUnsignedLongSet storage

As of CONTROLLER-2011 we are using TreeSets to track ranges in ImmutableUnsignedLongSet, which has a different characteristic from ImmutableRangeSet, which is using an internal array.

This can be wasteful for small and/or highly-fragmented sets, due to TreeSet being internally a Red-Black Tree.

Implement an array-based immutable NavigableSet, which we can use as storage for small(ish) arrays.