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	Fix mkSynTy
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Fix a nasty and long-lived bug in mkSynTy which meant that in:

	newtype A a = A a
	type B = A
	f :: B Double

the type (B Double) wasn't regarded properly as (A Double).  This bug
has lasted for a long time because the type inference engine is fairly
forgiving about missing the invariant that a TyConApp always looks
like one right at the top level.  In fact, it's so forgiving that I
don't know how to tickle this bug into showing up.  (It showed up in
5.02, becuase the context-simplification for 'deriving' is done by a
different engine as all the other context simplification.)

Still, the invariant should hold, and this fix makes it so.