The fix implemented to deal with primitive types auto-converting was
suppressing a series of legitimate unreachable warnings.  So we removed
that fix and tried to mirror the auto-conversion of the type, such that
the space intersection calculation ends up with the correct results.

In doing so we triggered reachability warnings being emitted by
sealed trait companion object (or enum) `ordinal` synthetic methods, in
some weird test cases.  So we added an `@unchecked` annotation to
suppress those.

Co-authored-by: Seth Tisue <seth@tisue.net>

In the event that the first case is unreachable, the logic descended to
`isSubspace(covered, prevs)` where both are the Empty Space, emitting
warnings.  Instead we implement reachability like the Scala 2 compiler
defines it:

    // a case is unreachable if it implies its preceding cases

…e fallback for type spaces