Function unescape_string 

pub fn unescape_string(
    data: &[u8],
    additional_escape_map: &HashMap<u8, TCodepoint>,
) -> Result<Vec<u8>, TCodepoint>

Decode an escaped grammar-literal byte string into its UTF-8 form.

Convenience helper layered on parse_next_utf8_or_escaped: walks the whole input, decoding each char or escape and re-encoding the resulting codepoints to UTF-8. Returns Err with the offending error sentinel on the first malformed char/escape.