iscollectable
if (!iscollectable(o)) return 0;
lua_longassert(!iscollectable(obj) || righttt(obj))
#define valiswhite(x) (iscollectable(x) && iswhite(gcvalue(x)))
#define gcvalue(o) check_exp(iscollectable(o), val_(o).gc)
lua_longassert(!iscollectable(obj) || \
(ttisdeadkey(gkey(n)) && iscollectable(key) &&
lua_assert(iscollectable(t1));