Integrate OpenSpiel

[dbravender]

I'm currently using davidADSP/SIMPLE#34 to train games. The library isn't worked on much and it's hard to get running given the older requirements.

Rather than fix/improve the existing implementation, many issues could be addressed (#2 #3, etc.) by porting https://github.com/deepmind/open_spiel implementation of ismcts to Dart.

Additionally, it would be nice to have a fast way to communicate between Dart and Python/C++ to use OpenSpiel to train neural networks to play games.

I want to avoid having to write game engines in C++ (for me it will take longer to integrate, write, debug and test than it would using Dart). Writing game engines in Python might work but it would most likely be slower and it would add a potentially difficult-to-maintain dependency on mobile devices.

[dbravender]

I'm not sure this is necessary anymore after fixing the issues with the neural-net-guided tree search the performance is much better.