Make the renamer so that the class ops on the LEFT HAND SIDE
of the bindings of an instance decl count as free variables
of that declaration.  E.g.

	instance Foo [a] where
	  op x  = ...
	  bop y = ...

Here, 'op' and 'bop' are now counted as free variables of
the decl.

This is vital, because the class decl for Foo might be imported,
and look like this:

	class Foo a where
	  op  :: a -> S
	  bop :: T -> a

and these might happen to be the only mentions of S and T
in the program.  Then we need to treat S and T as instance
gates for the purpose of hauling in further instance decls,
and the Right Way to do that is to announce that 'op' and
'bop' have been mentioned.

I also removed the (now obselete) rn_omit field in the
monad.