CLJ-1098: Implement IKVReduce and CollFold for nil

+ added tests

Signed-off-by: Rich Hickey <richhickey@gmail.com>
binary compatibility notes
[maven-release-plugin] prepare release clojure-1.5.0-RC3
[maven-release-plugin] prepare for next development iteration
impose once semantics on fabricated closures for e.g. loops
[maven-release-plugin] prepare release clojure-1.5.0-RC4
[maven-release-plugin] prepare for next development iteration
Restore {:as x} destructuring for empty lists [CLJ-1140]

Signed-off-by: Rich Hickey <richhickey@gmail.com>