Major revision for minimal gain...

I wrote GameboardLW (eliminating the need for GameboardCell) to create a
less memory-intensive representation that relies more on primitive types
than objects. This allowed OptimalRobotsPlayer to search another 2-3
levels of depth (total of 6-7 moves) before running out of space on the
heap (using 1GB). I annotated Gameboard and GameboardCell as
"deprecated." I also rewrote Enum types to make things cleaner, adding
RobotPiece and Goal, and annotating GoalType and PieceColor as
"deprecated."