Remove extra len calls in doubly-linked-list's methods

[amirsoroush]

Describe your change:

I reduced the number of calls to __len__ by storing the result into a local variable as it's currently a costly method( O(N) ).

To elaborate more: Suppose we insert an item at the tail like:

l = DoublyLinkedList()
l.insert_at_tail("foo")
l.insert_at_tail("boo")   # <------- Here

The second call to insert_at_tail causes __len__ to be called three times:

1. inside self.insert_at_nth(len(self), data) (because of delegating to insert_at_nth)
2. one at if not 0 <= index <= len(self) inside insert_at_nth
3. one at elif index == len(self) inside insert_at_nth

Same thing happens in delete_at_nth.

---

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• Fix a bug or typo in an existing algorithm?
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[dhruvmanila]

Makes sense, thanks!