Refactoring `vocab` module and fix potentially broken methods.

[filippodebortoli]

I opened the broken-tests-vocabulary branch on my fork. I introduced a few tests in the vocab module that are currently failing:

• vocab::test_is_xsd_datatype
• vocab::test_namespace_in_entity_for_iri
• vocab::test_to_built_in_entity

In addition, I left a few additional TODOs throughout the module:

• evaluate if vocab::WithIRI could be moved to model or potentially into a new iri module together with model::IRI and model::ForIRI
• evaluate if vocab::IRIString can be deprecated and replaced by model::IRI
• fix vocab::to_built_in_entity to make the corresponding test pass
• fix vocab::entity_for_iri to make the corresponding test pass
• fix vocab::is_xsd_datatype to make the corresponding test pass

Already present TODOs:

• implement Meta::all() for vocab::Vocab

I consider these to be potential bugs (hence the labels).