Add Compositor class as superclass of Arrow, in Control.Compositor

The Compositor class has two members:

class Compositor comp where
  identity :: comp a a
  (>>>) :: comp a b -> comp b c -> comp a c

with the obvious monoid. A number of useful types are Compositors but not Arrows.

Trac metadata

Trac field	Value
Version	6.6.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries/base
Test case
Differential revisions
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Blocking
CC
Operating system	Unknown
Architecture	Unknown