kiji-express: EXP-352. Column[Input/Output]Spec expose fields unnecessarily making it difficult to evolve.

issue: https://jira.kiji.org/browse/EXP-352
review: https://review.kiji.org/r/1544
commit: https://github.com/kijiproject/kiji-express/commit/ff7bc979a26e2ef2b4bfb9778351661102471b31