feat(#25): impl Iterator for LinkedList

[kul-sudo]

No description provided.

[slbsh]

dont make a LinkedList the iterator, instead make a new type:

impl<T> LinkedList<T> {
    pub fn iter(&self) -> Iter<'_, T> {
        Iter { node: self.front, len: self.len, _boo: PhantomData }
    }
}

pub struct Iter<'l, T> {
    node: Link<T>,
    len:  usize,
    _boo: PhantomData<&'l Node<T>>,
}

impl<T> Iterator for Iter<'l, T> {
    type Item = &'l T;

    fn next(&mut self) -> Option<&'l T> {
        /* do the logic */
    }
}