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.