Live coding of Ruby Chess v4 part 3.
* vim slowness fix (:00:45)
* refactoring from string to symbol for piece (01:27)
* piece.pawn? (04:30)
* Position[R: e4] syntax (07:20)
* move_str (output as algebraic notation) (12:24)
* move_str for pawn (21:00)
* move_str for king (38:00)
* possible_move_str (41:30)
