Into version for SecretString expose_secret

[tisonkun]

I found that we have a rewrite also. When do we release 0.9 for secrecy?

For this issue, I found:

impl<S> ExposeSecret<S> for Secret<S>
where
    S: Zeroize,
{
    fn expose_secret(&self) -> &S {
        &self.inner_secret
    }
}

But perhaps I'd like to move the inner secret in some case, and thus an into version can help to avoid copy.