move lua context out of dungeons[]

and out of save files so restore doesn't need to clear stale data.
Behavior should be the same as before, except that when entering
the endgame branch and discarding the main dungeon and its other
branches, lua theme context is now discarded for those too.

include/dungeon.h

@@ -67,7 +67,6 @@ typedef struct dungeon {   /* basic dungeon identifier */
     xchar dunlev_ureached; /* how deep you have been in this dungeon */
     int ledger_start,      /* the starting depth in "real" terms */
         depth_start;       /* the starting depth in "logical" terms */
-    lua_State *themelua;   /* themerms compiled lua */
 } dungeon;
 
 /*