pub fn val<T>(&'a mut self) -> Option<T>
    where
        T: FromZvalMut<'a>,
    {
        self.zval
            .as_mut()
            .and_then(|zv| T::from_zval_mut(zv.dereference_mut()))
    }
Attempts to retrieve the value of the argument.
This will be None until the [`ArgParser`] is used to parse
the arguments.
src/args.rs