swift234

Ensure internal ObjC classes aren't exposed in archives.

We currently specify a few internal ObjC classes.  This commit ensures that they can not be accidentally exposed (and thus need to be supported) into the future. Luckily, NSData and NSDictionary/NSMutableDictionary already specify classForCoder in a general way. NSCharacterSet does not.

This commit also establishes unit tests to ensure all three types do not regress from this into the future.

Also, for completeness, it ensures classForArchiver and classForKeyedArchiver return appropriately as well.