C API for node iteration forthcoming

[molpopgen]

@jeromekelleher pointed out that the next C API release will have C API functions for iterating over nodes in a tree. We should explore using this API, replacing the existing rust code for preorder traversal if it works out. If it does work, then we can get other traversal orders "for free".

The cost is more unsafe code blocks.

[jeromekelleher]

Two things worth bearing in mind @molpopgen

1. There's only preorder and postorder currently (have issue for inorder: Add inorder traversal to C API tskit#1797)
2. There's a O(n) memory overhead. If Rust has nice iteration abstractions to avoid this that are efficient, then might be worth sticking with whatever you have (if you have anything!)

[molpopgen]

Thanks--that's good to know. I'm okay with those two orders, as they're the most obviously useful ones. The overhead is presumably a stack? The rust implementation of preorder builds root/node stacks, and so is O(n) extra memory, too.

[jeromekelleher]

There's that, but also the array you need to malloc to get the results back. This example is idiomatic: https://github.com/tskit-dev/tskit/blob/b8d664712244f2eaeaab6fb4d19f84001c676738/c/tskit/trees.c#L3594

[molpopgen]

Ah, I see. Yeah, the rust iterator has a "current node" field that it "yields" until we hit the end.

Still, the new API will be an easy path for postorder until I/someone reach the activation energy for a pure rust version.

[molpopgen]

@jeromekelleher -- working on this in #186