Our abstract interfaces are more useful if we guarantee that
implementations conform to certain rules. Namely, we want to ensure
that objects implementing interfaces don't expose new public
attributes that aren't part of the interface. That way, as long as
consumers don't access "internal" attributes (those beginning with
"_") then (in theory) objects implementing interfaces can be swapped
out and everything will "just work."

We add a test that enforces our "no public attributes not part
of the abstract interface" rule.

We /could/ implement "interface compliance detection" at run-time.
However, that is littered with problems.

The obvious solutions are custom new and init methods.
These rely on derived types actually calling the parent's
implementation, which is no sure bet. Furthermore, new and
init will likely be called before instance-specific attributes
are assigned. In other words, they won't detect public attributes
set on self.dict. This means public attribute detection won't
be robust.

We could work around lack of robust self.dict public attribute
detection by having our interfaces implement a custom getattribute,
getattr, and/or setattr. However, this incurs an undesirable
run-time penalty. And, subclasses could override our custom
method, bypassing the check.

The most robust solution is a non-runtime test. So that's what this
commit implements. We have a generic function for validating that an
object only has public attributes defined by abstract classes. Then,
we instantiate some peers and verify a newly constructed object
plays by the rules.