Add support for simple CTEs

[lmcmicu]

We should allow structured selects (see #4) to be chained together in a simple way, so that the first items in the chain are interpreted as simple CTEs and the last item is the main query to execute. At this time we do not need to support more advanced CTEs defined using with recursive ... etc. We only need to support linking together Select statements defined in the simple way indicated in #4.