Add transducer arity to functions operating on maps

[yuhan0]

See discussion in #12

Add a 1-argument arity to the following functions, which returns a transducer consuming and producing a sequence of [key value] map-entry pairs, and can be used to construct a map using (into {} xform coll).

• map-kv
• map-keys
• map-vals
• filter-kv
• filter-keys
• filter-vals
• remove-kv
• remove-keys
• remove-vals
• index-by

The second commit refactors out repeated code into a private completing-rf function, if needed this can be omitted or squashed into the previous one.

[yuhan0]

Tests for the new transducer arities haven't been written yet, is it alright to simply duplicate the test cases in a (into {}) form? eg.

(deftest test-map-keys
  (is (= (map f coll)
         expected)))

=> 

(deftest test-map-keys
  (testing "maps"
    (is (= (m/map-keys f coll)
           expected)))
  (testing "transducers"
    (is (= (into {} (m/map-keys f) coll)
           expected))))

[yuhan0]

I pushed the requested changes and added simple "duplicate" tests of the existing ones (they all pass), let me know if you'd like the commits to be squashed in a particular way.

[weavejester]

I've considered this, but on reflection I don't think I want the map functions in Medley to support transducers.