Added support for the TRUE and FALSE keywords

Also added support for evaluating constant expressions in select.c, which is a
trivial memcpy.

Signed-off-by: Morten Hustveit <morten.hustveit@gmail.com>
select.c: Actually interpret the WHERE expression

Previously, we simply parsed and recorded the WHERE expression, but did not use
it for anything.  Now it's evaluated in the same fashion as the output
expression, tested for BOOLEAN type and used to filter each row.

Signed-off-by: Morten Hustveit <morten.hustveit@gmail.com>