Add PolyKinds to Data.GADT.Compare.TH

[ryantrinkle]

This allows deriveGCompare to generate instances of GCompare for types of kind (k -> ) rather than just ( -> *)

[ryantrinkle]

I just added support for GHC 8 here; it's very ugly, but it's not clear to me how to do it substantially better.

[mokus0]

does the GHC 8 support get any cleaner now that your patch to th-extras is on hackage?

[ryantrinkle]

I don't think it does, at least not in a terribly obvious way. What's happening is that there was an extra field added to InstanceD, which we need to pattern match against and then reconstruct.

For DataD and DataInstD, we could potentially use accessors for all of the fields we need and pattern match with DataD {} and DataInstD {}, respectively, provided those accessors exist somewhere.

[mokus0]

ok, no need to spend a bunch of effort prettifying the solution to an ugly problem.