Logic for mapcat and ParallelBuffer

@patrickwmcgee I've been testing the ClojurePlatform against the api_test.clj and almost all of them pass!  To test I've been copying over the tests from api_test.clj to my own test file...obviously your macro will be much better than this hacked approach.

Also, I know you have logic for mapcat as well, which I'm happy to use, I just wanted to get this stuff wired up first.


tuple-sort is a function.  It's defined higher up in the file, [specifically here](https://github.com/bhiles/cascalog/blob/bsh-more-clj-engine-capabilities/cascalog-core/src/clj/cascalog/clojure/platform.clj#L123).