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 libraries@haskell.org:
http://www.haskell.org/pipermail/libraries/2012-August/018268.html
Fell off the radar. No complaints in the 2 week period since being
brought up again:
http://www.haskell.org/pipermail/libraries/2013-May/019845.html
Patch from Edward Kmett (essentially.)
Authored-by: Edward Kmett <ekmett@gmail.com>
Signed-off-by: Austin Seipp <aseipp@pobox.com>