s_mp_rshd
void s_mp_rshd(mp_int *mp, mp_size p); /* right-shift by p digits */
s_mp_rshd(&x, used / 2);
s_mp_rshd(mp, d / DIGIT_BIT);
s_mp_rshd(&q, USED(m) - 1); /* q1 = x / b^(k-1) */
s_mp_rshd(&q, USED(m) + 1); /* q3 = q2 / b^(k+1) */