- Reversing a change that was made in 0.9, the "singleton" nature

- Reversing a change that was made in 0.9, the "singleton" nature
of the "constants" :func:`.null`, :func:`.true`, and :func:`.false`
has been reverted.   These functions returning a "singleton" object
had the effect that different instances would be treated as the
same regardless of lexical use, which in particular would impact
the rendering of the columns clause of a SELECT statement.
fixes #3170