Refactor, improve, and document the deriving mechanism

This patch does a fairly major clean-up of the code that implements 'deriving.

* The big changes are in TcDeriv, which is dramatically cleaned up.
  In particular, there is a clear split into
	a) inference of instance contexts for deriving clauses
	b) generation of the derived code, given a context 
  Step (a) is skipped for standalone instance decls, which 
  have an explicitly provided context.

* The handling of "taggery", which is cooperative between TcDeriv and
  TcGenDeriv, is cleaned up a lot

* I have added documentation for standalone deriving (which was 
  previously wrong).

* The Haskell report is vague on exactly when a deriving clause should
  succeed.  Prodded by Conal I have loosened the rules slightly, thereyb
  making drv015 work again, and documented the rules in the user manual.

I believe this patch validates ok (once I've update the test suite)
and can go into the 6.8 branch.

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