allow getting node using usize as an index.

[coderedart]

I need to pass around the NodeID via ffi functions. And because its an opaque rust struct, I decided to simply get the usize and use that as the node_id for the sake of simplicity.

But, NodeID also has a version, making it impossible to get the original NodeID from just the usize.

It would be nice if the arena had a function to get the current NodeID at the index usize (or None if it doesn't exist).