pub fn string_field<T: Peek + Next>(name: &str, stack: &mut T) -> Result<String, XmlParseError> {
    start_element(name, stack)?;
    let value = characters(stack)?;
    end_element(name, stack)?;
    Ok(value)
}
return a string field with the right name or throw a parse error
rusoto/core/src/proto/xml/util.rs