indextree 4.2.0

Arena based tree structure by using indices instead of reference counted pointers
Documentation 
# indextree

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## Arena based tree structure with multithreading support

This arena tree structure is using just a single `Vec` and numerical identifiers
(indices in the vector) instead of reference counted pointers. This means there
is no `RefCell` and mutability is handled in a way much more idiomatic to Rust
through unique (&mut) access to the arena. The tree can be sent or shared across
threads like a `Vec`. This enables general multiprocessing support like
parallel tree traversals.

### Example usage

```rust
use indextree::Arena;

// Create a new arena
let arena = &mut Arena::new();

// Add some new nodes to the arena
let a = arena.new_node(1);
let b = arena.new_node(2);

// Append a to b
assert!(a.append(b, arena).is_ok());
assert_eq!(b.ancestors(arena).into_iter().count(), 2);
```