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.

src/do.c

@@ -1420,6 +1420,8 @@ boolean at_stairs, falling, portal;
         update_mlstmv(); /* current monsters are becoming inactive */
         if (nhfp->structlevel)
             bufon(nhfp->fd);       /* use buffered output */
+    } else {
+        free_luathemes(TRUE);
     }
     save_mode = nhfp->mode;
     nhfp->mode = cant_go_back ? FREEING : (WRITING | FREEING);