WIP ImmutableLongOpenHashSet
Finish ImmutableLongOpenHashSet and write test
WIP ImmutableVector
Add VectorIterator
Make ImmutableVector classes private
Add VectorNode.updated
Use ImmutableVector everywhere
ImmutableVector.updated: Use System.arraycopy instead of Array.updated

This results in a 4x speedup for ImmutableVector.updated.
More efficient ImmutableVector creation
VectorIterator: Mark fields private[this], avoiding a cast
Fix bug in ImmutableVector.size
Remove recursion in InternalNode.apply
Remove variance annotations for specialization
Add ImmutableBitSet
Use Immutable{Vector,BitSet} in ImmutableLongOpenHashSet
Use ImmutableLongOpenHashSet in IndexedRDDPartition
Use Immutable{LongOpenHashSet,BitSet} in VertexPartition
Implement efficient insertions in multiput
Fix performance bug in ImmutableVector.size
Add support for deletion