Add alternative subject representations for RDF

[RickMoynihan]

We currently support a turtle like nested vector syntax for tersely creating RDF, however when you wish to manipulate RDF as data this representation isn't ideal.

We should add first class support for an RDF representation that looks more like this:

{::grafter/subject #grafter/uri "http://foo.com/id/rick"
   rdf:a foaf:Person}

We should also allow serialisation/deserialisation into this format also; i.e. make things like statements work on it.

Finally this is likely a useful format for in memory graphs and data models to use. In particular for creating clojure.spec generators to generate arbitrary RDF conforming to a model.