The API for writing new visitors for the query parse tree is too complex

[pdowler]

The top level navigator requires three separate components that visit different parts of the query (expressions, references, and from-items)... It might be much simpler to have a single monolithic Visitor that covers all the visit() methods even if that was a large number of methods.

Issues to consider:

• re-use of navigator components is currently sketchy depending on whether they have/accumulate state or not
• would be good to allow sub-select in the from clause, but that means recursion support (especially in the SelectListExtractor which has to percolate column metadata out to the top-level select)