Added a proper implementation of TreeSet, based on a Persistent set implementation.

classpath/java/util/Cell.java

@@ -0,0 +1,24 @@
+package java.util;
+
+public class Cell <T> {
+  public T value;
+  public Cell<T> next;
+  
+  public Cell(T value, Cell<T> next) {
+    this.value = value;
+    this.next = next;
+  }
+
+  public String toString() {
+    StringBuilder sb = new StringBuilder();
+    sb.append("(");
+    for (Cell c = this; c != null; c = c.next) {
+      sb.append(value);
+      if (c.next != null) {
+        sb.append(" ");
+      }
+    }
+    sb.append(")");
+    return sb.toString();
+  }
+}