[LIB] Add new linked list functions to libft

[itislu]

• ft_lstadd_back_eff()
• ft_lstnew_back_eff()
• ft_lstdup()
• ft_lstsize_d()

The efficient versions of ft_lstadd_back expect a tail pointer of the list to avoid traversing the whole list.
I made those function in an older, unmerged branch in February already.

[LeaYeh]

I think *_eff is really confused, why not just replace the logic?

[itislu]

Because it requires a tail pointer as a third argument.
The caller has to provide that, which would require to add that parameter to all calls of ft_lstadd_back() and ft_lstnew_back().

[LeaYeh]

I see the interface change, but i will suggest you to use efficient but not the brief