Fix the treatment of lexically scoped kind variables (Trac #8856)
The issue here is described in Note [Binding scoped type variables] in TcPat. When implementing this fix I was able to make things quite a bit simpler: * The type variables in a type signature now never unify with each other, and so can be straightfoward skolems. * We only need the SigTv stuff for signatures in patterns, and for kind variables.
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