waterhouse's implementation of AVL trees

Thanks to Pauan for bumping this back to our attention:
http://arclanguage.org/item?id=14181. I'll add his changes next.
Pauan's test harness for AVL trees

From http://arclanguage.org/item?id=14181. No fixes yet. Haven't
actually tried running this commit or the last.
test passes; thanks Pauan and waterhouse

http://arclanguage.org/item?id=14181