Fix subtyping of null and refined types.
Drop NotNull class and associated code.
Fix #248: Class cannot inherit from refinement types

We now check that classes do not inherit from refinement types
(unless they are an encoding of parameterized types), nor from
& or | types.
Remove obsolete mentions of the NotNull class
One more test case, coming from #214.
Fixed test
Merge pull request #278 from dotty-staging/fix/#248-inherit-refinements

Fix #248: Class cannot inherit from refinement types
Merge pull request #277 from dotty-staging/change/drop-not-null

Change/drop not null
Fix #239 - handling of package objects

References to `.package` are now also inserted if the
accessed member comes from a class inherited by a package
object.