We define interfaces (using zope.interface) for main resource classes
and refer to these interfaces (as dotted names) in API views' context
instead of registering these views against the resource classes
directly.

The idea is to make it possible for downstream applications to register
their own resource classes that could be selected by standard API views.

Interfaces have attributes and methods declared based on what appears to
be needed on API views. We use interface inheritance to reduce
duplication (e.g. the IMapped interface is a base for all interface but
IRoot; IRelatedEntities is a base for IRelatedEntity).

Notice that this somehow conflicts with the Mappable abstract class that
was defined previously in resources module; so we drop this abstract
class and replace it by an IMapped interface.