Bug fix.  The hash function shouldn't depend on the way the JDK decides to split the computation.
These tests have already been moved into the PersistentMap tests since they are universally applicable to other maps.
Added javadoc to Ref.
Mark local variables as final.
New Red-Black tree implementation.  Only one class with a flag telling us what type on node it is.  No virtual calls for dispatch.  Much faster but uses more memory.