Method add

tests/relacy/freelist.cpp:23–32

    FreeList() : freeListHead(nullptr) { }

    inline void add(N* node)
    {
    	// We know that the should-be-on-freelist bit is 0 at this point, so it's safe to
    	// set it using a fetch_add
        if (node->freeListRefs.fetch_add(SHOULD_BE_ON_FREELIST, std::memory_order_acq_rel) == 0) {
            // Oh look! We were the last ones referencing this node, and we know
            // we want to add it to the free list, so let's do it!
     	   add_knowing_refcount_is_zero(node);
        }
    }

    inline N* try_get()
    {