Substring( const basic_nsAReadableString<CharT>& aString, PRUint32 aStartPos, PRUint32 aSubstringLength )
Substring( const nsReadingIterator<CharT>& aStart, const nsReadingIterator<CharT>& aEnd )
Substring( const basic_nsAReadableString<CharT>& aString, PRUint32 aStartPos, PRUint32 aSubstringLength )  {    return nsPromiseSubstring<CharT>(aString, aStartPos, aSubstringLength);  }