Proposal: Add Compositor class as superclass of Arrow

[haskell at list.mightyreason.com]

Conal Elliott wrote:
> I'm delighted to see these interfaces being explored.  Question: why
> separate fan-out (&&&) from pair?  Do you know of type constructors that
> have fst & snd but not &&&?  Similarly for CategoryAssoc.   - Conal
> 

Naively, I would venture that one might not want to be able to consume the input
more than once.  There were papers that modeled quantum mechanics/computation
using arrows, but required care care not to have any fan out.

Practically, I have no idea if this is a good example of such a category.

-- 
Chris