Re: extend pgbench expressions with functions

Hello Robert,

Here is a v13 and a small test script.
 - change names to random(), random_exponential() and random_gaussian()   I find them too long, but if the committer want that I cannot help   it:-)
 - more comments, especially about the expression evaluation &   type system.
 - improved documentation, in particular to include suggestions by Tomas   Vondra about clarifying explanations about the gaussian &   exponential random generators, and clear references from \setrandom   to the \set expressions.
 - still just one patch, because removing double would mean removing the 2   exponential & gaussian random functions which require a double   argument.
   Note that I started with one small patch for adding the infrastructure,   but then Heikki requested more functions including double type stuff to   illustrate the point, then Robert asks to break it back, going forward   and backward is tiring...
 - still "lousy" *debug functions, because I found them useful for   debugging and testing, really. It is easy to remove them, but I would   advise against doing that as it would make debugging an expression   much less straightforward.

-- 
Fabien.