Note that there is not currently any navigation from a type to its
instances, except for Enumeration. I tried using Enumeration but
ran into HTML formatting issues, commented that out for now.
This is for cases where we want to give multiple types in the schema config, without verbosity.
This update also takes care to normalize the use of "rdf:type" in @property to
create the exact same internal structure, i.e. canonicalize to http://schema.org/typeOf.
Intent is to avoid subtle bugs where we have two kinds of type triple depending on which
piece of RDFa syntax is used.
Required for #218 and #130 cleanup of meta-model.