Declaration.__add__ easily produces inconsistent RO

[jamadden]

Noticed while working on zopefoundation/zope.schema#80.

It simply takes itself as the __bases__, and whatever is unique to its second argument it tacked on, ignoring their relative order. If the two arguments partially overlap, this is almost guaranteed to produce an inconsistent RO.

[jamadden]

This is an example of the kind of thing that would fail in strict C3 mode:

from zope.interface import Interface
from zope.interface import implementedBy
from zope.interface import implementer

class IBase(Interface):
    pass

class IDerived(IBase):
    pass

@implementer(IBase)
class Base(object):
    pass

d = implementedBy(Base) + IDerived