oaep: support non-string labels

[baloo]

This rework oaep to support non-string labels.
One use-case is encryption of secrets in TPM.

https://trustedcomputinggroup.org/wp-content/uploads/TPM-2.0-1.83-Part-1-Architecture.pdf#page=297
Section B.4 RSAES_OAEP

For RSA keys protecting a secret value (such as, an encryption key or a session secret), the L parameter
is a byte stream, the last byte of which must be zero, indicating the intended use of the encrypted value. 

That would look like:

    let encrypted_seed = {
        let padding = Oaep::new_with_label::<EkHash, _>(b"IDENTITY\0".to_vec());
        let enc_data = ek_public
            .encrypt(&mut rng, padding, &random_seed[..])
            .expect("failed to encrypt");
        enc_data
    };

[baloo]

I'm pulling this PR in parallaxsecond/rust-tss-esapi#563

[tarcieri]

@baloo can you rebase?