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/decl.h

@@ -949,6 +949,7 @@ struct instance_globals {
     int mhitu_dieroll;
 
     /* mklev.c */
+    genericptr_t luathemes[MAXDUNGEON];
     xchar vault_x;
     xchar vault_y;
     boolean made_branch; /* used only during level creation */