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/concurrent/ConcurrentMap.java

@@ -0,0 +1,23 @@
+/* Copyright (c) 2008-2014, Avian Contributors
+
+   Permission to use, copy, modify, and/or distribute this software
+   for any purpose with or without fee is hereby granted, provided
+   that the above copyright notice and this permission notice appear
+   in all copies.
+
+   There is NO WARRANTY for this software.  See license.txt for
+   details. */
+
+package java.util.concurrent;
+
+import java.util.Map;
+
+public interface ConcurrentMap<K,V> extends Map<K,V> {
+  public V putIfAbsent(K key, V value);
+
+  public boolean remove(K key, V value);
+
+  public V replace(K key, V value);
+
+  public boolean replace(K key, V oldValue, V newValue);
+}