Enhance type-safe object access via explicit conversions.

To allow for type-safe access of the internal object data, users can now
register convertible-to relations with the runtime. This is useful when
announcing derived classes of a polymorphic hierarchy and then requesting
to cast an object to the corresponding base class.

The free functions make_convertible/is_convertible register/check a
convertible-to relation.

In the end, this featurism is just a mediocre compensation for the lack of
introspection in C++.

Closes #23.