(#1008): Clean up the items that are visible in the public API
This is mainly to avoid internal items from appearing in the public Rust API documentation.
Also, I've changed the return value in AcceptLanguage::parse() -> Result<AcceptLanguage, String>
- whereas it used to return an AcceptLanguageError
for the error case before. This is because the calling app cannot really do anything with the error value, it means that the app passed in an invalid Accept-Language header payload.
Fixes #1008 (closed).