proposal #2254: have Control.Arrow re-export (>>>) and (<<<)

[Don Stewart]

ross:
> A previous proposal:
> 
> 	http://hackage.haskell.org/trac/ghc/ticket/1773
> 
> split the Arrow class, making (>>>) and (<<<) functions in
> Control.Category.  This breaks programs that use Control.Arrow.
> 
> To avoid some of the breakage, the proposal is that Control.Arrow should
> re-export (>>>) and (<<<), imported from Control.Category.

This patch has been applied (xmonad was broken due to the change, so I
thought it best to act).

-- Don