Function long_key

pub fn long_key(username: &str, key: &str, realm: &str) -> [u8; 16]

create long key.

key = MD5(username “:” OpaqueString(realm) “:” OpaqueString(password))

let buffer = [
    0x3eu8, 0x2f, 0x79, 0x1e, 0x1f, 0x14, 0xd1, 0x73, 0xfc, 0x91, 0xff,
    0x2f, 0x59, 0xb5, 0x0f, 0xd1,
];

let key = faster_stun::util::long_key("panda", "panda", "raspberry");
assert_eq!(key, buffer);