Treat the (~) type constructor a bit specially

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.