Fibonacci impl

Added classic recursive, tail recursive and iterative implementation of the Fibonacci sequence
I think there is no performance impact in this case. The only difference is semantic (the meaning of the program).

As you correctly said, `move_iter` will disallow reusing the vector, which can be desirable in some occasions. I think it is in this case a good alternative. My only concern is that it is slightly more complex than just using `iter` (from a beginner's perspective).

On the other hand, it is good to show that a move iterator exists, as it is something that a beginner will have to learn some day. So I actually think it is ok!