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.