first unboxed sums patch is ready for reviews

[Ömer Sinan Ağacan]

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