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
Two new tests

1) Verify we survive illegal infinite paths. Closes #91.
2) Verify we handle fbounds in and types correctly.