Improve scripting language in pgbench

Improve scripting language in pgbench

Added:
 - variable now might contain integer, double, boolean and null values
 - functions ln, exp
 - logical AND/OR/NOT
 - bitwise AND/OR/NOT/XOR
 - bit right/left shift
 - comparison operators
 - IS [NOT] (NULL|TRUE|FALSE)
 - conditional choice (in form of when/case/then)

New operations and functions allow to implement more complicated test scenario.

Author: Fabien Coelho with minor editorization by me
Reviewed-By: Pavel Stehule, Jeevan Ladhe, me
Discussion: https://www.postgresql.org/message-id/flat/alpine.DEB.2.10.1604030742390.31618@sto