Signature type variables must not be instantiated with tycons
An egregious bug in the type checker meant that it was possible for a "signature type variable" (a MetaTv of SigTv form) to be instantatiated with a type-constructor application. This destroys the invariant for SigTv. The fix is easy; adding the predicate TcType.isTyConableTyVar Fixes Trac #1153
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