pgbench more operators & functions

Hello,

Here is a simple patch which adds a bunch of operators (bitwise: & | ^ ~, 
comparisons: =/== <>/!= < <= > >=, logical: and/&& or/|| xor/^^ not/!) and 
functions (exp ln if) to pgbench. I've tried to be pg's SQL compatible 
where appropriate.

Also attached is a simple test script.

Some kind of continuations in \ commands would be a good thing.

-- 
Fabien.