Initial stub-out of class methods.
Build up DIFFS logic.
Sketch out movement logic.
Add render method.
Decide on approach for diff logic.
Add row and column numbers to render.
First version of populate method.
Fix bugs to attain initial output.
populate and render both have the proper orientation.
Add piece move logic.
Pass basic move test.
Calculate valid moves and implement moving logic.
Fix stupid rendering bug.
diffs now point left and right. Prepare for moves to accept l, r commands.
Add InvalidMoveError class.
Add variant error.
Infrastrure for perform_moves! method.
Implement dup in support of move calculator.
Add valid_move_seq?
Pieces only accepts symbol input.