Suggestion: Panic when appending to node that has self node as child.

[mpawelski]

Hi,
thanks for writing this nice library.
For some reason I thought I could use indextree as a graph structure. Later I realized that this library is just for tree structures. And I got such strange behavior:

        let arena = &mut Arena::new();

        let start = arena.new_node("start");
        let a = arena.new_node("A");
        let b = arena.new_node("b");
        let c = arena.new_node("c");

        start.append(a, arena);
        a.append(start, arena);
        start.append(b, arena);
        b.append(start, arena);
        assert_eq!(start.children(arena).into_iter().count(), 2);

        a.append(c, arena);
        c.append(a, arena);

        assert_eq!(start.children(arena).into_iter().count(), 2); //FAILED: count is 1

I guess doing "proper" validation to check if I'm building graph is difficult. For example this gives infinite look:

        let arena = &mut Arena::new();

        let a = arena.new_node("a");
        let b = arena.new_node("b");
        let c = arena.new_node("c");

        a.append(b, arena);
        b.append(c, arena);
        c.append(a, arena);

        assert_eq!(a.children(arena).into_iter().count(), 1);
        assert_eq!(a.descendants(arena).into_iter().count(), 3); // infinite loop

But in my case just panic when trying to "connect" nodes to each other would give me earlier feedback that I'm using this library in a wrong way. I propose that for second append the code below should panic (I hope it's easy to check it, I didn't look at the source code)

        let arena = &mut Arena::new();

        let a = arena.new_node("a");
        let b = arena.new_node("b");

        a.append(b, arena);
        b.append(a, arena); //IMO this should panic

        assert_eq!(a.children(arena).into_iter().count(), 1);
        assert_eq!(a.descendants(arena).into_iter().count(), 2); // infinite loop

BTW, the example code on https://crates.io/crates/indextree is not compiling (assert!(a.append(b, arena).is_ok()); line)