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	Better grouping for ty/cls decls
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When finding mutually-recursive groups of type and class decls,
we shouldn't include classes mentioned in a deriving clause as
edges. E.g.

	class Eq a where ...

	data Bool = True | False deriving( Eq )

Eq and Bool are not mutually recursive.

The edges are computed by RnHsSyn.tyClDeclFVs, so we remove the
derivings from there.

There a consequential fix in RnSource.rnSourceDecl