---------------------
	Dealing with deriving
	---------------------

I spent a ridiculously long time peering at a bug report whereby
a 'deriving' clause sent GHC 5.02.1 into a loop.  It was all to
do with allowing instances like

	instance Foo a b => Baz (T a)

(Notice the 'b' on the left which does not appear on the right.)

I realised that it's hard for the deriving machinery to find a
fixpoint when these sort of instance decls are around.  So I
now constrain *derived* instance decls not to have this form;
all the tyvars on the left must appear on the right.

On the way I commoned up the previously-separate tcSimplify
machinery for 'deriving' and 'default' decls with that for
everything else.   As a result, quite a few files are touched.

I hope I havn't broken anything.