Stand-alone deriving declarations added

[Simon Peyton-Jones]

| Good points. The only remaining argument that I can see is then that
| it would make Greg Fitzgerald's suggestion to allow this:
| 
| deriving Data, Typable for Person, Team, Department, Company
| 
| difficult to do as far as I can see. Though we could still allow
| multiple derivings in one declaration, just not the cartesian product
| version above:
| 
| deriving Data Person, Typeable Person, Data Team, Typeable Team
| deriving Data Department, Typeable Department, Data Company, Typeable
| Company

Yes, I'd be happy with requiring the programmer to enumerate the
instances s/he wants, rather than having an implicit Cartesian product.

S