This struct takes the query text and a root rule that will work as the
starting point to generate the expression tree that represents the query.
This expression tree has a virtually identical structure to the formed
by the rules themselves, with the only difference that ?/+/* rules may
have none or many nodes representing a single rule.

The parser does not do anything beyond a syntactic interpretation of the
query, the expression tree nodes contain a pointer to the rule that
generated them, plus start/end points of the text in the query, so an
upper layer can do the actual ontology validations, and generate the SQL.

The two entry points are tracker_sparql_parse_query/update, that correspond
to the QueryUnit/UpdateUnit entry points defined in the SPARQL grammar.