An implementation of persistent sorted maps and sets based on AVL
trees which can be used as drop-in replacements for Clojure's
built-in sorted maps and sets based on red-black trees. Apart from
the standard sorted collection API, the provided map and set types
support the transients API and several additional logarithmic time
operations: rank queries via clojure.core/nth (select element by
rank) and clojure.data.avl/rank-of (discover rank of element),
"nearest key" lookups via clojure.data.avl/nearest, splits by key
and index via clojure.data.avl/split-key and
clojure.data.avl/split-at, respectively, and subsets/submaps using
clojure.data.avl/subrange.