`append_at(other: Self, at: usize)` in O(n) time

[orxfun]

This method can be implemented in O(n) time:

• O(n) to find the position to append (at)
• O(1) to append

It would reflect the recursive nature of the list.

[orxfun]

Now with NodeIndex made available, the following methods could also be implemented to execute in O(1):

• append_next_to(&mut self, other: Self, node: NodeIndex)
• append_prev_to(&mut self, other: Self, node: NodeIndex)