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/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 */
 };