Comonads

[nomicflux]

Initial implementation of Comonads for review and revision

[jnape]

This PR is so high-quality it nearly brings me to tears. Let me look it over to offer thoughtful feedback, but this is the new gold standard for PRs.

[jnape]

@nomicflux discovered a bug in the process of putting this PR together in javac that effectively prevents Tuples from being Comonad, related to the same reason #2 isn't yet in lambda. He's already submitted the bug report to Oracle, and I'm going to watch the issue to get a sense of their urgency in addressing it and it's feasibility, and go from there. If it doesn't look good in terms of getting the bug patched anytime soon, I may still just merge this in; it would just be unfortunate to have a situation where Tuples, which are de facto examples of Comonads, not able to be Comonads.

[nomicflux]

Javac issue is logged at https://bugs.java.com/bugdatabase/view_bug.do?bug_id=JDK-8225508.

Test case for the issue is here: https://gist.github.com/nomicflux/11ea8a5a9cc2f4c9a0cc73a26517124a.