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