Stand-alone deriving declarations added

[Bjorn Bringert]

Hi all,

I have just pushed some patches which add support for stand-alone  
'deriving' declarations. The main motivation for this is to allow you  
to use the instance deriving mechanism for data types and newtypes  
declared in other modules, for example if you want to use generics  
with existing code that does not derive Typeable and Data, or want to  
use Show for debugging.

The syntax is:

deriving Class for Type

and for multi-parameter type classes:

deriving (Class t1 ... tn) for Type

You can't derive multiple classes in a single deriving declaration,  
but you can have multiple deriving declarations for the same type.

I have added a small section to the type system extensions part of  
the Users' Guide, and a number of test cases.

This is a direct result of the GHC hackathon.

/Björn