Disallow Optional Classes at the Slice Level

[InsertCreativityHere]

This PR adds a check to the Slice parser which will emit an error if you mark a type as optional if that type uses classes. It also fixes the last few random places where we were using optional classes in the tests.

Now that this is complete, it also re-enables cross testing of the csharp/Ice/optional test. And to keep the tests in parity, it adds back a portion of a test which was removed from C#. Both of these were 'leftovers' from #2114.