first unboxed sums patch is ready for reviews

Hi all, I'm almost done with the unboxed sums patch and I'd like to get some reviews at this point. https://phabricator.haskell.org/D2259 Two key files in the patch are UnariseStg.hs and RepType.hs. For the example programs see files in testsuite/tests/unboxedsums/ In addition to any comments about the code and documentation, it'd be appreciated if you tell me about some potential uses of unboxed sums, example programs, edge cases etc. so that I can test it a bit more and make sure the generated code is good. Thanks, Omer