Never unify a SigTyVar with a non-tyvar type (fixes Trac #7786)
This unwanted unification was happening in the zonking phase which un-flattens type-function applications (TcMType.zonkFlats, try_zonk_fun_eq). The main unifier is careful to make the check, but I'd forgotten it here. That in turn led to a very confusing error message.
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