rust schema tooling: kdl-schema-check

[boringcactus]

I've put together a very rough draft of a schema verifier that can be used as a command line utility or as a library within some higher-level utility, and I kept the schema types themselves as a separate library so that other tools that use KDL Schema don't have to reinvent the wheel there.

Both pieces of this are still in progress, and schemas that use more features than the KDL schema schema itself won't be handled well, so my next step is either to write better examples or to start working on generating Rust types from a KDL Schema definition automatically (which I want to do eventually anyway, and which seems like a really good way to make sure my Rust types match the schema schema).

Unfortunately, there appears to be a Pijul bug that means the kdl-schema-check repository shows up as empty, but conveniently the source is visible on docs.rs.

Thanks a bunch @tailhook for knuffel - that crate has the exact things I needed from it to make my life easy.