Function lfsr_step 

pub fn lfsr_step(register: u64, feedback: u64) -> u64

One step of the Galois LFSR: shift right, and fold in the feedback polynomial when the bit shifted out was set.

Bijective on the non-zero registers, which is what makes the shuffle above it a permutation rather than a hash. Zero is a fixed point and is kept out of range by the 1-based normalization.