Treat the (~) type constructor a bit specially
when kind-checking in Core Lint. It's unusual becuase it is poly-kinded; for example (~) Int a and (~) Maybe b are both ok. We don't want the full generality of kind polymorphism (yet anyway) so these changes in effect give (~) its own private kinding rule. It won't work right if (~) appears un-saturated, and Lint now checks for that too.
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