Allow Pieces to assign themselves to a board.
Fix Piece#perform_slide
Fix Piece#perform_jump
Consolidate Piece#jump_moves and Piece#slide_moves into Piece#possible_moves
Protect Piece#sandwich_square and Piece#possible_moves
Change 'protected' helper methods in Piece to private methods
Create Piece#dup_board
Write framework for Piece#perform_moves
Complete Piece#perform_moves! and update valid_move_seq?
Create InvalidMoveError
Create custom_errors and InvalidMoveError