Add functions for external collection hashing

External collections wishing to implement hasheq appropriately must follow the advice at http://clojure.org/data_structures#hash. To simplify the implementation (and avoid unwanted dependencies on the internal Murmur3 class), add two new functions hash-ordered-coll and hash-unordered-coll that provide a proper collection hasheq over entire collections.

Patch: clj-1348-3.patch (fixes [k v])