REORG: tools: promote the debug PRNG to more general use as a statistical one

REORG: tools: promote the debug PRNG to more general use as a statistical one

We frequently need to access a simple and fast PRNG for statistical
purposes. The debug_prng() function did exactly this using a xorshift
generator but its use was limited to debug only. Let's move this to
tools.h and tools.c to make it accessible everywhere. Since it needs to
be fast, its state is thread-local. An initialization function starts a
different initial value for each thread for better distribution.