Pairs transducers

xforms makes really easy and efficient to work with pairs.

Clojure maps do not use map entries internally, pairs are only allocated when you call find or iterate over a map. reduce-kv avoids pair allocation by expecting a reducing function which accepts three arguments ([acc k v]).

xforms introduces the KvRfable protocol. It allows a transducer to determine whether its downstream reducing function accepts pairs (as discrete arguments) or not.