StringExt function as_der_bytes referenced in readme but not exposed to users

[sinking-point]

In the readme, there's an example of SignedDataVerifier usage:

// .unwrap() used for example purposes only
let root_cert = "apple-root-cert-in-base-base64-format"; // https://www.apple.com/certificateauthority/AppleRootCA-G3.cer
let root_cert_der = root_cert.as_der_bytes().unwrap(); // Use `base64` crate to decode base64 string into bytes 

let verifier = SignedDataVerifier::new(
    vec![root_cert_der], // Vector of root certificates
    Environment::Sandbox, // Environment
    "app.superapp.apple".to_string(), // Bundle id
    Some(12345678), // App id
);

let payload = "signed-payload";
let decoded_payload = verifier.verify_and_decode_notification(payload).unwrap();

In this example, as_der_bytes from the StringExt trait (defined in utils.rs) is used. However, in lib.rs:

mod utils;

The utils module isn't made public. As far as I can tell, there's no way for me to import the trait needed for me to use as_der_bytes.

Please could you make the trait possible to import? Or otherwise, remove it from the example.

Thank you for making this crate public, by the way.