Fibonacci impl

Added classic recursive, tail recursive and iterative implementation of the Fibonacci sequence
on second reflection you're right, it's not immediately evident why tester would need to return a bool and you should not have to inspect the places where it's called to find out why. I changed tester not to return anything and the actual test to:
```
    fns.iter()
        .advance(|&f| {tester(f); true});
```
please let me know if you think it's more readable that way. Thanks!