This patch adds a new node type Boolean, to go alongside the "value"
nodes Integer, Float, String, etc. This seems appropriate given that
Boolean values are a fundamental part of the system and are used a lot.
Before, SQL-level Boolean constants were represented by a string with
a cast, and internal Boolean values in DDL commands were usually
represented by Integer nodes. This takes the place of both of these
uses, making the intent clearer and having some amount of type safety.