Expose `fy_node_mapping_sort` and `fy_node_sort`

[jonathanrainer]

Hi,

First of all thank you so much for making such a brilliant crate, I'm really impressed with it's usability and it feels a really nice niche in the Rust YAML ecosystem if you want YAML parsing as well as comment preservation etc!

Now I'm more than happy to do the legwork myself, and so I recently published a change to libfyaml that exposes fy_node_mapping_sort as previously it was an internal function here. I'd really like to expose that and fy_node_sort to this library, and to that end I've published a PR on fyaml-sys here which pulls through the new version of the C-library and adds an env var to build.rs.

I also have a PR ready to go on this repo that utilises that change in fyaml-sys and adds the necessary functions and tests to expose sort_at and sort_mapping_at.

So, would it be possible to review and merge the PR on fyaml-sys and from there review my other PR once I can raise it?

Again thanks for a brilliant crate and I'd love to be able to contribute if I can !