reduce可以在Java映射上工作，但reduce-kv不行
`
user=> (def clojure-map {1 2 3 4})
'user/clojure-map
user=> (def java-map (java.util.HashMap. {1 2 3 4}))
'user/java-map
user=> (reduce (fn [sum [k v]] (+ sum k v)) 0 java-map)
10
user=> (reduce-kv + 0 clojure-map)
10
user=> (reduce-kv + 0 java-map)
IllegalArgumentException No implementation of method: :kv-reduce of protocol: #'clojure.core.protocols/IKVReduce found for class: java.util.HashMap clojure.core/-cache-protocol-fn\
(core_deftype.clj:544)
`
在常规reduce中解构参数非常简单，但也有性能影响。以下示例展示了使用实现此补丁中java.util.Map的reduce-kv实现的7倍速度提升。
`
user=> (def big-clojure-map (into {} (map #(vector % %) (range 10000))))
'user/big-clojure-map
user=> (def big-java-map (java.util.HashMap. big-clojure-map))
反射警告，/tmp/form-init7130245387362554027.clj:1:19 - 无法解析对java.util.HashMap构造函数的调用。
'user/big-java-map
user=> (defn reduce-sum [m] (reduce (fn [sum [k v]] (+ sum k v)) 0 m))
'user/reduce-sum
user=> (defn reduce-sum-kv [m] (reduce-kv (fn [sum k v] (+ sum k v)) 0 m))
'user/reduce-sum-kv
user=> (time (dotimes [_ 1000] (reduce-sum big-java-map)))
"Elapsed time: 2624.692113 msecs"
nil
user=> (time (dotimes [_ 1000] (reduce-sum-kv big-java-map)))
"Elapsed time: 376.802454 msecs"
nil
`
