pow support for pgbench

Hi,

I've written a small patch to add support for pow() in pgbench.

The main reason behind it is that I'm currently using a shell call to do it
which takes between 2-10 ms that can be a big percentage of the time taken
by the whole transaction. For example (shortened):

latency average = 11.718 ms
 - statement latencies in milliseconds:
         2.834  \setshell POWER2  awk 'BEGIN {p=2^ARGV[1]; print p }'
:ZOOM_CURRENT
         8.846  SELECT
ST_AsBinary(ST_Simplify(ST_SnapToGrid("the_geom_webmercator",:SNAP),
:SIMPLIFY)) AS geom FROM

I've also updated the related docs and added some tests. Please let me know
if I'm missing anything.

Regards,
*Raúl Marín Rodríguez*
carto.com