Use struct to store position.
Add method Board::is_outside_board.
Fix method Board::make_walking_move.
Add method Board::is_possible_move.
Implement few operators in struct pos_t.
Fix Board bugs.

- add constructor to struct pos_t;
- fix returning result of method Board::is_outside_board;
- check for walls in method Board::is_possible_move;
Check walls when pawn walks.
Remove unused method Console::read_move.