Fibonacci impl

Added classic recursive, tail recursive and iterative implementation of the Fibonacci sequence
@utkarshkukreti: The way you propose would avoid allocations, but I was trying to exploit the implementation of Show on Vec.
My version prints:
```
recursive implementation:
 [0, 1, 1, 2, 3, 5, 8, 13, 21, 34]
```

the version with the inner loop would print:
```
recursive implementation: 0
recursive implementation: 1
...
```

I could do it with something like:
```
for (f, desc) in fns.move_iter() {
    println!("{}", desc);    
    for i in range(0u64, 10) {
        print!("{} ", f(i));
    }
    println!();
```
but (in my personal taste) the code starts looking uglier that way... What do you think?