Implement CREATE TABLE ... AS SELECT borrowing code from SubSelect

 and from SELECT ... INTO ... support code.
Allow NOT, IS NULL, IS NOT NULL in constraints.
Define unionall boolean flag in SubSelect structure.
Implement row descriptors: (a, b, c) = (x, y, z).
Change IS TRUE, IS FALSE, etc. to expressions using "=" rather than
 function calls to istrue() or isfalse() to allow optimization.
Force type for TRUE and FALSE to bool.