Add processor state, term constructor, and triple constructors. Chang…

…e reifiedTriple to reifingTriple. (#62)

* Add processor state, term constructor, and triple constructor for `annotationBlock`, `tripleTerm`, and `reifier`.
* Extract the `annotationBlock` production from the `annotation` production.
* Use "reifiedTriple" instead of "reifyingTriple". This better describes how the production relates to the `reifying triple` definition in RDF Concepts.
* `reifier` is optional, not zero or more, in `reifiedtriple`.
* Use `verb` rather than `predicate` in the `reifiedTriple` EBNF production.
* Add other productions that bind to curSubject.
* Improve naming consistency.
* In Processor State explain that "records" creates a stack of saved values of these variables.
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Co-authored-by: Ted Thibodeau Jr <tthibodeau@openlinksw.com>
Co-authored-by: Olaf Hartig <olaf.hartig@liu.se>
Co-authored-by: Andy Seaborne <andy@apache.org>