`toolz.merge_with` not lazy, breaks with Clojure interface

Clojure's core.merge-with takes a binary operator

(defn add [a b]
  (+ a b))  ; yes I know this is silly, I wanted an explicit binary operator

(merge-with add [{:a 1} {:a 2} {:a 3}])
;; {:a 6}

Toolz merge_with takes a reduction operation

>>> merge_with(sum, [{'a': 1}, {'a': 2}, {'a': 3}])
{'a': 6}

This is convenient from a Python perspective because operations like sum are more familiar than binary operators like add. However this is not lazy and requires all dicts to be read in at once.

I tried a solution in https://github.com/pytoolz/toolz/pull/127 but gave up.

Start at the merge_with entry point and compare its current reduction behavior with Clojure's core.merge-with. Review the abandoned pull request #127 before defining the expected lazy behavior and compatibility tests; done means the intended input pattern works without requiring all dictionaries to be read at once.