Write class skeletons
Piece#slide_moves, other small edits

 - Improve out Piece#initialize
 - Board#[](pos), place_piece, and remove_piece
Render methods, other small edits

 - fix Piece::DELTAS
 - begin jump_moves
 - begin testing
Testing jump_moves

 - Add on_board? method
 - Still fiddling with jump_moves
Get jump_moves working, write board.dup
Everything looks like it's working
Board setup working
Perform_slide, perform_jump working

 - Simplify jump_moves
 - Write Board#play_turn
Write board dup
Fix bug in jump_moves
Looks like perform_moves! works

 - Edit king handling in perform slide / jump methods
 - Remove Board#place_piece
Write valid_move_seq?