implement pre-order traversal of node

[remodietlicher]

Hello! I'm playing around with your nice library to learn a bit of Rust. I found myself filtering and mapping the AST, for example to extract only a specific type of Node. After implementing my own filter function I noticed that the Iterator trait actually provides just that, all that's needed is the next function.

So here I implement a pre-order traversal of the Node-tree to create an Iterator over the AST. I show how it can be used on the to_string implementation.

Since I'm new to Rust, I was wondering if this sounds useful to you and if there is anything I could improve.

[wooorm]

Hey! Thanks for your patience, cool work!

This seems a bit similar to my preorder traversel utility here: https://github.com/wooorm/mdxjs-rs/blob/8cba71bcf926057ae4bdc5b67de69010a5621301/src/mdast_util_to_hast.rs#L1198-L1222. (I also had a mutable version there as well: wooorm/mdxjs-rs@f6b705f#diff-d1bf57e8534841c5ee7eda0ab2bcf89b943719c21ea89a48bf96e25a114e2f3f)

I’ve been meaning to turn the nodes into traits (see #27 (comment)). That way, we can have such utilities working on mdast and hast.
Perhaps that’s something you might be interested in working on? :)