Make Board references constant where possible to improve clarity and avoid unnecessary copies

[KvanTTT]

It looks like NNEvaluator::evaluate is a hot path and it would be nice to get rid of redundant copies. And it looks consistent because BoardHistory is also passed by const reference to evaluate.

[lightvector]

Thanks for the change! I had to check things over to verify that the board (and the hist) actually is const in places, but I think this looks good.

[KvanTTT]

Thanks for the merging!

Actually, I've discovered much more unnecessary copies (using CLang-Tidy). The things should be treated carefully, because in some places copy syntax is used intentionally and it's incorrectly to use const references. But they should be emphasized explicitly.

Also, I've found a lot of places where push_back can be replaced with emplace_back and other performance problems. I will create a meta-issue to track them and will prepare merge requests step by step.