we should also have a bootstrap clone method for attributes

otherwise we waste a lot of time (and potentially screw things up) by
trying to fiddle with metaclass compat and such
move a bunch of other stuff into mop::class::BUILD
better error messages for these tests
move all extra initialization for metaclasses into BUILD

none of it is necessary during bootstrapping (which never calls BUILD),
and this lets us always use the replaced mop::object::new in metaclasses