Fibonacci impl

Added classic recursive, tail recursive and iterative implementation of the Fibonacci sequence
@aochagavia if you don't mind, I'll just leave it as iter(). I would think that iter() is sort of the "default option" just on the basis that it's the one appearing on most examples and having the shorter name. 
You were surprised to see it used and, thinking about it, my first reaction seeing a move_iter() in someone else's code would be exactly like yours ("why move_iter() and not iter() here?"). Since iter() works here, I think it's better to avoid surprises and leave examples of move_iter() to cases where iter() would not work.