Extend core RDF mappings to support compound fields

Description

Most fields in Drupal core have just one value and this plays nicely with the way RDF mappings are structured in core: one mapping definition (predicates, datatype, callback) per field. However some fields in contrib have several values which need to be annotated in RDF, for example geolocation, Voting API, address field.

Requirements

- multiple values per field
- optionally allow for a nested resource generation (bnode by default, but possiblity to specify a URI callback). address is a good example where a bnode is usually used to group the street, zip, city etc.
- backward compatible with the core RDF mapping structure

one level of nesting ought to be enough for most field types, generally if more nesting is needed, proper references/entities would be used.

This would allow to extend the core text_with_summary field for example to have a different mapping for its 'value' and 'summary'.

Proposal

Nest an RDF mapping array inside a #mapping key. Example using some values from the geolocation module (ignore the predicates, some are made up!):

function placemodule_rdf_mapping() {
  return array(
    array(
      'type' => 'node',
      'bundle' => 'place',
      'mapping' => array(
        'rdftype' => array('geo:Place'),
        'title' => array(
          'predicates' => array('geo:name'),
        ),
        'description' => array(
          'predicates' => array('geo:description'),
        ),
        'location' => array(
          'predicates' => array('vcard:geo'),
          '#mapping' => array(
            'lng' => array(
              'predicates' => array('vcard:longitude'),
            ),
            'lat' => array(
              'predicates' => array('vcard:latitude'),
            ),
          ),
        ),
      ),
    ),
  );
}

Another example with address field:

function organizationmodule_rdf_mapping() {
  return array(
    array(
      'type' => 'node',
      'bundle' => 'organization',
      'mapping' => array(
        'rdftype' => array('foaf:Organization'),
        'title' => array(
          'predicates' => array('foaf:name'),
        ),
        'address' => array(
          'predicates' => array('vcard:adr'),
          '#mapping' => array(
            'rdftype' => array('vcard:Home'),
            'thoroughfare' => array(
              'predicates' => array('vcard:street-address'),
            ),
            'postal_code' => array(
              'predicates' => array('vcard:postal-code'),
            ),
            'locality' => array(
              'predicates' => array('vcard:locality'),
            ),
            'country' => array(
              'predicates' => array('vcard:country-name'),
            ),
          ),
        ),
      ),
    ),
  );
}