An ADQL metadata inference and query morphing library.

As a rule, I've tried to keep as much in plain strings as possible to
avoid having gargantuous trees when we're really only interested in
rather few "hot spots".  Of course, the downside is that tree processing
becomes a bit tedious in places.

What's where:

* The grammar, parsing into basically nothing: grammar.py
* Nodes, functioning as callbacks for the grammar and building a query
  structure: nodes.py
* Processing of nodes, actual tree building: tree.py
* Metadata inference: annotations.py
* Creation of actual postgres queries from the results of the above
  modules: morphpg.py, morphhelpers.py; postproc.py belongs here
  to some extent, but it's tree manipulation that's probably desirable
  independent of the target language.