Use NonZeroUsize inside NodeId

[SimonSapin]

This allows Option<NodeId> to be no larger than usize, and saves 32 bytes of memory per node.

To allow IDs to be used directly as indexes, the Vec<Node<T>> storage has an uninitialized item at index 0. Safety-wise, this "infects" the entire crate: every access to this Vec needs to make sure to skip that first (non-)item.

[SimonSapin]

Safety-wise, this "infects" the entire crate

At first I started moving the Tree struct into a small module where the vec field would be private, so that this unsafety could be "more contained". But then I realize this module would need a non-trivial amount of abstraction in order to support the rest of the crate, particularly iterators. Still, at ~1k lines I feel that this crate is just above the size where this kind of pervasive unsafety is manageable, so maybe this abstraction would pay off. What do you think?

[SimonSapin]

#16 was an alternative that offset IDs by 1 instead of having an uninitialized Vec item.

[causal-agent]

I'm not really happy with the amount of unsafe in this crate, but I'm not sure I'd be happier with creating more abstraction inside it either. At least for now, this looks fine. I'll reconsider if more changes need to be made in the future.