Function serve 

pub fn serve(
    dir: &Path,
    key_file: &Path,
    tokens_file: &Path,
    listen: &str,
    ready: impl FnOnce(&str),
) -> Result<(), String>

bynk registry serve: serve dir with the publish API, signing with key_file and authorising with tokens_file, on listen. ready is told the address actually bound, once listening. Runs until the process ends.