implement ConcurrentHashMap and AtomicReferenceArray

This is the simplest possible ConcurrentHashMap I could come up with
that works and is actually concurrent in the way one would expect.
It's pretty unconventional, being based on a persistent red-black
tree, and not particularly memory-efficient or cache-friendly.  I
think this is a good place to start, though, and it should perform
reasonably well for most workloads.  Patches for a more efficient
implementation are welcome!

I also implemented AtomicReferenceArray, since I was using it in my
first, naive attempt to implement ConcurrentHashMap.

I had to do a bit of refactoring, including moving some non-standard
stuff from java.util.Collections to avian.Data so I could make it
available to code outside the java.util package, which is why I had to
modify several unrelated files.

classpath/java/util/HashSet.java

@@ -63,7 +63,7 @@ public class HashSet<T> extends AbstractSet<T> implements Set<T> {
   }
 
   public String toString() {
-    return Collections.toString(this);
+    return avian.Data.toString(this);
   }
 
   private static class MyIterator<T> implements Iterator<T> {