AES-CTR-DRBG

DRBG implementation based on AES-256.

Usage:

``` use aesctrdrbg::DrbgCtx;

fn main() {

// personalization string must be min. 48 bytes long
let p = vec![48, 0];

// get entropy from somewhere, f.e. /dev/random
let entropy: [u8; 48] = [0x04; 48]; // don't use that!

let mut drbg = DrbgCtx::new();
drbg.init(&entropy, p);

// get 10 bytes
let mut out = Vec::new();
out.resize(10, 0);
drbg.get_random(&mut out);

println!("{:?}", out);

} ```