Kill off zipTopTCvSubst in favour of zipOpenTCvSubst
As Bartosz has discovered, the invariants for substitutions were wrong, and in particular the "mkTop...Subst" and "zipTop..Subst" functions were building substitutions that didn't obey even the old invariants. This patch kills of the bogus zipTopTCvSubst in favour of the more robust zipOpenTCvSubst. I tripped over this because my upcoming patch (concerning SetLevels, Trac #11330) triggered an ASSERT failure in the substitution well-formedness assertion in TyCoRep.
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