Proposal: Add Compositor class as superclass of Arrow

[Ashley Yakeley]

I wrote:
> http://hackage.haskell.org/trac/ghc/ticket/1773
> (darcs patch attached to ticket)
...
> Proposal period: two weeks, until 10-27

Consensus was to add this under the name Category:

class Category cat where
   id :: cat a a
   (.) :: cat b c -> cat a b -> cat a c

id and (.) would not replace those in the Prelude. Category would become 
a superclass of the existing Arrow class.

Updated code, pushed patches to base, closed ticket "fixed". Thanks 
everyone!

-- 
Ashley Yakeley