Implement `ZeroizeOnDrop` for `SecretKeyVT`

[dima74]

Would be nice to leverage zeroize crate (already in dependencies) to zeroizing secret keys on drop.

E.g. similar to ed25519-dalek:

impl Drop for SecretKeyVT {
    fn drop(&mut self) {
        self.0.zeroize();
    }
}

impl ZeroizeOnDrop for SecretKeyVT {}