The super-duper fix metaclass update.

The super-duper fix metaclass update.

Basically, this implements the whole "if two classes differ by roles,
reconcile the roles" algorithm.

Along the way, this also fixes some bugs in
_fix_metaclass_compatibility, where it could lose things like roles on
a metaclass's attribute metaclass if the parent class's metaclass was
a subclass of said metaclass (ugh, this is hard to talk about).