Function deadlock_right

fn deadlock_right(burrow: &Burrow) -> bool

Mirror image situation to deadlock_left where a D amphipod could block others.

For example:

    #############
    #.......D...#
    ###.#.#.#A###
      #.#.#.#B#
      #.#.#.#C#
      #.#.#.#D#
      #########

The hallway has room for the top two amphipods but the D prevents the bottom two from returning to their home burrow.