In the standard library, we should use explicit universe variables for
universe polymorphic definitions.
Users that want to declare universe polymorphic definitions but do not
want to provide universe level parameters should use
Type _
or
Type*
Perhaps, we should add an option to disable this new feature.
Remark: this commit makes commit 46d418a redundant.
I'm keeping 46d418a because we may retract this commit in the future.