* lisp/play/landmark.el: Use lexical-binding and avoid `intangible'.
(landmark--last-pos): New var.
(landmark--intangible-chars): New const.
(landmark--intangible): New function.
(landmark-mode, landmark-move): Use it.
(landmark-mode): Remove properties.
(landmark-plot-square, landmark-point-square, landmark-goto-xy)
(landmark-cross-qtuple):
Don't worry about `intangible' any more.
(landmark-click, landmark-point-y): Same; and don't assume point-min==1.
(landmark-init-display): Don't set `intangible' and `point-entered'.
(square): Remove. Inline it instead.
(landmark--distance): Rename from `distance'.
(landmark-calc-distance-of-robot-from): Rename from
calc-distance-of-robot-from.
(landmark-calc-smell-internal): Rename from calc-smell-internal.