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	Be a bit more liberal when slurping instance decls
	--------------------------------------------------

Functional dependencies have (as usual) made things more complicated

Suppose an interface file contains
	interface A where
	  class C a b | a->b where op :: a->b
	  instance C Foo Baz where ...

Now we are compiling
	module B where
	  import A
	  t = op (v::Foo)

Should we slurp the instance decl, even though Baz is nowhere mentioned
in module B?  YES!  Because of the fundep, the (C Foo ?) part is enough to
select this instance decl, and the Baz part follows.

Rather than take fundeps into account "properly", we just slurp
if C is visible and *any one* of the Names in the types
This is a slightly brutal approximation, but most instance decls
are regular H98 ones and it's perfect for them.

Changes:

  HscTypes:
	generalise the types of GatedDecl a bit

  RnHiFiles.loadInstDecl, RnHiFiles.loadRule, RnIfaces.selectGated:
	the meat of the solution

  RdrName, OccName etc:
	some consequential wibbles