In Rust, we have Rc<Node> for the main refcounted node struct.

We export that to C by boxing as Box<Rc<Node>>, since an Rc<Node>
normally lives in the stack - that is, the Rc is in the stack, and the
Node and its refcount are in the heap.

When we ref() a node, we return a new Box:

  Box::into_raw (Box::new (node.clone ()))

This means that in C, "x = rsvg_node_ref (y)" gives you back a pointer x
that is different from y!  Both refer to the same Node in the Rust
world, and after that the node has one more refcount, as expected.

When we unref() a node, we expect you to call "x = rsvg_node_unref (x)".
This returns NULL, so your x=NULL after that, i.e. the node that was in
x is not accessible anymore.