* Add ttSubject for subject of tripleTerm and remove NIL.

* Fix some references to non-existent term definitions.
* Spec updates (with placeholders) for reified triples and annotations.
* Update grammar for annotations and triple terms using `~` prefix for reifier.
* Remove extraneous statement on curSubject and curTriple.
* Note on old vs new reification.
* Fix duplicate example identifier.
* Remove reference to "asserted triple", and fix reference to "annotation" production.
* Update to use "reification" and "rdf:reifies" instead of "triple occurence" and "rdf:nameOf".
* Fix object of tripleOccurence to be `object`, not `ttObject`.
* Update description and processing instructions for triple terms, triple occurrences, and annotations.
* Grammar updates for triple terms and occurrences.

Co-authored-by: Ted Thibodeau Jr <tthibodeau@openlinksw.com>
Co-authored-by: Pierre-Antoine Champin <pierre-antoine@w3.org>