Evaluate Position instead of (Position, Move)

[lrozenblyum]

We should RnD on whether it's now better to evaluate just Position.

Benefits

1. performance (now for every evaluator we calculate new Position (Position, move)
2. ability to define more strong 'symmetry' via assert
   (e.g. for each move
   eval(Position) = 1 - eval( Position.toMirror() )

Caused by #293

See initial idea
#130 (comment)