Add direct support RDF4j models

[RickMoynihan]

http://docs.rdf4j.org/javadoc/latest/?org/eclipse/rdf4j/model/package-summary.html

We should extend core grafter protocols to RDF4j models to support to-statements add etc.

Additionally we should define some new protocols to support

• match which should essentially be the models .filter
• models can have a prefix map associated with them, so we should have a protocol to get prefixes. This could be extended to queries, potentially repositories etc. We should add better prefix support also Compact URI Expansion grafter-vocabularies#1.
• Add support for converting construct results directly into a model.
• Support loading rdf files directly into a model, perhaps we should add a parallel to ->statements for ->model as a protocol?

Considerations:

• Immutability. The model API isn't immutable, so we may want to provide a grafter deftype for ImmutableModel, it should be set like etc. Our current position is not to convert query results etc into immutable grafter types by default though, so probably best to leave this as an optional extra. Though the grafter functions for models could perhaps emphasise/assume read-only/immutability?
• Also see this utility class for useful functions: http://docs.rdf4j.org/javadoc/latest/org/eclipse/rdf4j/model/util/Models.html

NOTE also: use of models is similar to the local readstore pattern, though without the ordered collections of identifiers.