Deleted obsolete type choices from QueryResult.types. Now uses DbTypes enum.
Added Step interface to SqlStatement. SqliteBack now uses new DbTypes enum.

* Removed code from sqlite backend for handling unordered fields in Datum
  objects.

* SqlStatement has a Step and Get[Int/Double/Text] methods for walking through
  the results of a query

* SqliteBack now uses the Step interface for all queries - avoiding the
  horrible, lossy from string conversion before.

* Updated SqliteBack tests to create non-ragged, consistent Datums for the
  table(s) created.
Added Step interface to SqlStatement. SqliteBack now uses new DbTypes enum.

* Removed code from sqlite backend for handling unordered fields in Datum
  objects.

* SqlStatement has a Step and Get[Int/Double/Text] methods for walking through
  the results of a query

* SqliteBack now uses the Step interface for all queries - avoiding the
  horrible, lossy from string conversion before.

* Updated SqliteBack tests to create non-ragged, consistent Datums for the
  table(s) created.