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/sun/misc/Unsafe.java

@@ -74,6 +74,8 @@ public final class Unsafe {
 
   public native int arrayBaseOffset(Class arrayClass);
 
+  public native int arrayIndexScale(Class arrayClass);
+
   public native long objectFieldOffset(Field field);
 
   public native void park(boolean absolute, long time);