Function bitcoin_muhash::addnextract2

pub fn addnextract2(c0: &mut Limb, c1: &mut Limb, a: &Limb, n: &mut Limb)

| Add limb a to [c0,c1]: [c0,c1] += a. Then | extract the lowest limb of [c0,c1] into | n, and left shift the number by 1 limb. |