Function string_to_codepoints 

pub fn string_to_codepoints<N: DomNavigator>(
    _context: &mut DynamicContext<'_, N>,
    args: Vec<XPathValue<N>>,
) -> Result<XPathValue<N>, XPathError>

fn:string-to-codepoints($arg as xs:string?) as xs:integer*

Converts a string to a sequence of codepoints.