Enable PolyKinds for Control.Category.
This change will allow us to make various new things instances of
Category, such as promoted tuples.
Original discussion from email@example.com:
Fell off the radar. No complaints in the 2 week period since being
brought up again:
Patch from Edward Kmett (essentially.)
Authored-by: Edward Kmett <firstname.lastname@example.org>
Signed-off-by: Austin Seipp <email@example.com>