Fibonacci impl

Added classic recursive, tail recursive and iterative implementation of the Fibonacci sequence
I am still a little bit dubious with this one, because here `advance` is used as if it was a `for_each` function. I think it would be clearer to use a `for` loop, because the `advance` function is meant for other kind of use case.

I have opened a PR (https://github.com/mozilla/rust/pull/14911) to add a `for_each` function to the `Iterator` trait. If it gets merged then we could use it here! `fns.iter().for_each(|f| tester(f));` seems perfect for this use case.