GC: use simple balanced tree instead of AVL tree

doc/lib.txt

@@ -63,6 +63,9 @@ Collections and algorithms
   Implementation of a queue. The underlying implementation uses a ``seq``.
 * `intsets <intsets.html>`_
   Efficient implementation of a set of ints as a sparse bit set.
+* `critbits <critbits.html>`_
+  This module implements a *crit bit tree* which is an efficient
+  container for a set or a mapping of strings.
 * `sequtils <sequtils.html>`_
   This module implements operations for the built-in seq type
   which were inspired by functional programming languages.