PolyKinds, Control.Category and GHC 7.6.1

[Simon Peyton-Jones]

| FWIW PolyKinds in 7.4 is rough, and I had experiences where enabling it
| led to compile failures in downstream modules, so this wouldn't
| necessarily have been painless. Hopefully with 7.6 it will be.

PolyKinds is not an advertised feature of 7.4, so you should absolutely not rely on it working in 7.4.  But it IS an advertised feature for 7.6 and should work flawlessly.  (If not, yell.)

Simon