Don't use an extraneous Arc, it adds an additional atomic inc/dec for ev...

...ery .fetch_add()

Timings after:

```
fun-with-threads cmr$ time ./rust_counter_atomics
20000000

real	0m0.891s
user	0m3.073s
sys	0m0.017s
fun-with-threads cmr$ time ./atomics
Final value of counter is: 20000000

real	0m0.939s
user	0m3.485s
sys	0m0.014s
```

I ran it a few times and got different results, I assume it's just the throttling on this not-plugged-in macbook air.