Reduce warnings

[ujh]

cargo test now doesn't have any warnings anymore and cargo build only has two.

[coveralls]

Coverage decreased (-0.49%) to 84.03% when pulling 5ce50c0 on reduce-warnings into c5144b9 on master.

[coveralls]

Coverage decreased (-0.59%) to 83.92% when pulling c1ba864 on reduce-warnings into c5144b9 on master.

[ujh]

Actually, now there's only one warning. It's for new_chain_liberties_greater_than, but I think you wanted to use that one @iopq, right?

[iopq]

I will know if I use it or not when I actually implement the code for saving atari stones.

[ujh]

OK, I'll leave it in then and you can remove it at some later point if you don't need it.