pub fn ct_add_u64(acc: &mut [u8], n: u64)
acc = (acc + n) mod 2^(8·acc.len()) for a small integer n, big-endian, in place. Used by Hash_DRBG to fold in the reseed_counter (SP 800-90A §10.1.1.4 step 6).
acc = (acc + n) mod 2^(8·acc.len())
n
reseed_counter