Made Pipes into Kleisli arrows.

Since Pipes were exactly Kleisli arrows anyway, I changed their types
from a -> m b to Kleisli m a b.

Change summary:

* Turned Pipes into Kleisli arrows.
* Replaced >=> with >>> as Pipe composition.
* Turned Control.Monad.FunctionGraph.switch into a Pipe.
Split FunctionGraph.hs into ParArrow.hs and Arrow.Utils.hs

Reorganized Control.Monad.FunctionGraph, splitting it into two pieces:
Control.Arrow.Utils, containing only functions pertaining to Pipes and
Control.Arrow.ParArrow, containing the functions which pertain to
ParPipes (renamed into ParArrows).

Also rewrote the module documentation to be less bombastic.