Quantify unfixed kind variables in CUSKs
This is a small change in user-facing behavior. When we
have a unification variable left over in a CUSK, we previously
would issue an error. But, we can just quantify. This also
teaches kcLHsQTyVars to use quantifyTyVars instead of its own,
ad-hoc quantification scheme.
Fixes Trac #15273.
test case: polykinds/T11648b