Improving C++ API

Changed the Piece and Board classes slightly so that I can make an efficient C++ agent.

I plan to make SilverFish as good as I possibly can.
Rather than writing 6 different mediocre sample agents like I did last year, I will just focus on this one.

Technically I can't enter my own competition, so SilverFish is still just a sample agent.

Anyway it still just defaults to random moves at the moment.