remove pi as well
Re-use the RNG.
This significantly improves the timing as can be seen below. Maybe
something can be gained from switching to floats if we want to.
```
% time ./pi
π = 3.13939
./pi  0.03s user 0.02s system 93% cpu 0.045 total

% time ./pi_approximation
Pi calculated with 1000000 steps:
3.13918
./pi_approximation  0.05s user 0.01s system 79% cpu 0.076 total
```