Implement CollReduce/IReduce on sorted maps and sets

Attached patch implements CollReduce in Clojure and IReduce in ClojureScript on AVLMap and AVLSet for a faster reduce with fewer intermediate allocations.

Rough benchmark in Clojure 1.5.1, Oracle Java 8, Ubuntu 16.04, Haswell i7

The with-init reduce implementation (e.g. (reduce conj [] coll)) is modelled on your existing reduce-kv implementation for maps and trivially transformed for sets.

The no-init reduce implementation (e.g. (reduce + coll)) is a bit funky. I use the internal function select to find the rank=0 node, extract its value to use as an init, then walk over the avl tree a second time to do the reduction. I avoid duplicating the first item in the reduction with an identical? check on the current node against the rank=0 node. Once I hit the rank=0 node again, I revert back to normal with-init reduction to drop the identical? check. Let me know if you see a better approach. However, I expect no-init reduction to be quite rare in practice, so I didn't try very hard to avoid walking part of the tree twice e.g. by keeping an internal node stack or something.

The transient versions of avl-map and avl-set are currently not reducible or seqable at all, so I didn't implement CollReduce/IReduce on them either.