Add an LFSR PRNG for bench

Add an LFSR PRNG for bench

The PRNG introduced by this commit is a Galois-XNOR LFSR, which can
generate unique pseudorandom numbers. Also, seeded with the same
seed, it will produce the same sequence.

This implementation is driven by the needs of XSEG for a blazing fast
random generator. Initial tests show that it's capable of producing 2^38
unique pseudorandom numbers in less than 10 minutes (which equals to 2
nanoseconds per number).