replace mersenne twister by pcg

[Reini Urban]

http://www.pcg-random.org/
See branch pcg

First without inlining 6% slower, now with inlining regained the 6% (5a39798)
pcg quality is better, but only 64bit.

http://xoroshiro.di.unimi.it/xoroshiro128plus.c is broken.