Function extract 

pub fn extract<H: Hasher>(ikm: &[u8], salt: Option<&[u8]>) -> Hash

HKDF extract step: PRK = HMAC-Hash(salt, IKM).

If salt is None, a string of H::OUTPUT_SIZE zero bytes is used.

Example

use crypto::hkdf;
use crypto::sha2::Sha256;

let prk = hkdf::extract::<Sha256>(b"input key material", Some(b"salt"));