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.

src/nhlua.c

@@ -839,6 +839,11 @@ lua_State *L;
             return nhl_push_anything(L, ustruct[i].type, ustruct[i].ptr);
         }
 
+    if (!strcmp(tkey, "inventory")) {
+        nhl_push_obj(L, g.invent);
+        return 1;
+    }
+
     nhl_error(L, "Unknown u table index");
     return 0;
 }