Do not use Class for IdentifierNamespace identification

The object model for namespaces in yang-model-api is awkward:
1. it requires Map<K, V>, where there are two classes of users: iterators and lookups
2. it requires weird abstract classes and Class<?> objects being passed around
3. it is a universally required, but there are a number of statement classes which do not care about namespaces

Revisit the definitions here, so that we replace access through EffectiveStatement.get(Class<? extends IdentifierNamespace>) by dedicated methods for each namespace.