Implement benchmark mode

This PR implements a benchmark mode, with the following API:
```
./iomrascalai -m pps [-s SIZE (default = 9) -r RUNTIME (default = 30s)]
```

2 small changes were made:
-> Playout.run() now takes a ref to a game, and not a game by-move.
-> Rewrote some of iomrascalai.rs, I think the purpose is clearer this way, and it fights less with options/matches (but if you don't like it, we can go back to the other way).