pgbench: Support double constants and functions.

The new functions are pi(), random(), random_exponential(),
random_gaussian(), and sqrt().  I was worried that this would be
slower than before, but, if anything, it actually turns out to be
slightly faster, because we now express the built-in pgbench scripts
using fewer lines; each \setrandom can be merged into a subsequent
\set.

Fabien Coelho