Rework stairs structure

Use a linked list to store stair and ladder information, instead
of having fixed up/down stairs/ladders and a single "special" (branch)
stair.

Breaks saves and bones.

Adds information to migrating objects and monsters for the dungeon
and level where they are migrating from.

include/decl.h

@@ -75,16 +75,6 @@ struct dgn_topology { /* special dungeon levels for speed */
 #define sokoend_level           (g.dungeon_topology.d_sokoend_level)
 /* clang-format on */
 
-#define xdnstair (g.dnstair.sx)
-#define ydnstair (g.dnstair.sy)
-#define xupstair (g.upstair.sx)
-#define yupstair (g.upstair.sy)
-
-#define xdnladder (g.dnladder.sx)
-#define ydnladder (g.dnladder.sy)
-#define xupladder (g.upladder.sx)
-#define yupladder (g.upladder.sy)
-
 #define dunlev_reached(x) (g.dungeons[(x)->dnum].dunlev_ureached)
 
 #include "quest.h"
@@ -726,10 +716,7 @@ struct instance_globals {
     int y_maze_max;
     int otg_temp; /* used by object_to_glyph() [otg] */
     int in_doagain;
-    stairway dnstair; /* stairs down */
-    stairway upstair; /* stairs up */
-    stairway dnladder; /* ladder down */
-    stairway upladder; /* ladder up */
+    stairway *stairs;
     int smeq[MAXNROFROOMS + 1];
     int doorindex;
     char *save_cm;
@@ -754,7 +741,6 @@ struct instance_globals {
        number of shots, index of current one, validity check, shoot vs throw */
     struct multishot m_shot;
     dungeon dungeons[MAXDUNGEON]; /* ini'ed by init_dungeon() */
-    stairway sstairs;
     dest_area updest;
     dest_area dndest;
     coord inv_pos;
@@ -765,9 +751,6 @@ struct instance_globals {
     boolean mrg_to_wielded; /* weapon picked is merged with wielded one */
     struct plinemsg_type *plinemsg_types;
     char toplines[TBUFSZ];
-    struct mkroom *upstairs_room;
-    struct mkroom *dnstairs_room;
-    struct mkroom *sstairs_room;
     coord bhitpos; /* place where throw or zap hits or stops */
     boolean in_steed_dismounting;
     coord doors[DOORMAX];