pgbench - add pseudo-random permutation function

Hello,

This patch adds a pseudo-random permutation function to pgbench. It allows
to mix non uniform random keys to avoid trivial correlations between
neighboring values, hence between pages.

The function is a simplistic form of encryption adapted to any size, using
a few iterations of scramble and scatter phases. The result is not
cryptographically convincing, nor even statistically, but it is quite
inexpensive and achieves the desired result. A computation costs 0.22 µs
per call on my laptop, about three times the cost of a simple function.

Alternative designs, such as iterating over an actual encryption function
or using some sbox, would lead to much more costly solutions and complex
code.

I also join a few scripts I used for testing.

--
Fabien.