Add lua object references

Whenever a lua script references a core struct obj, increment a counter
in the obj struct. Core code will not free the obj, if there are any
lua references pointing to it, just makes it free-floating.
When lua script ends, the lua gc will free the free-floating objects.

Also exposes u.inventory to lua.

Breaks save and bones compat.

include/extern.h

@@ -1675,6 +1675,7 @@ E int FDECL(l_selection_register, (lua_State *));
 
 /* ### nhlobj.c ### */
 #if !defined(CROSSCOMPILE) || defined(CROSSCOMPILE_TARGET)
+E void FDECL(nhl_push_obj, (lua_State *, struct obj *));
 E int FDECL(l_obj_register, (lua_State *));
 #endif
 

include/obj.h

@@ -119,6 +119,7 @@ struct obj {
     unsigned oeaten;        /* nutrition left in food, if partly eaten */
     long age;               /* creation date */
     long owornmask;
+    unsigned lua_ref_cnt;  /* # of lua script references for this object */
     struct oextra *oextra; /* pointer to oextra struct */
 };
 

include/patchlevel.h

@@ -14,7 +14,7 @@
  * Incrementing EDITLEVEL can be used to force invalidation of old bones
  * and save files.
  */
-#define EDITLEVEL 5
+#define EDITLEVEL 6
 
 #define COPYRIGHT_BANNER_A "NetHack, Copyright 1985-2019"
 #define COPYRIGHT_BANNER_B \