Caterpillar

I'm interrupting the current presentation on control flow for some breaking news: Caterpillar has a syntax now!

I chose to go with the example syntax I've been using over the last few days for a start. Nothing's final (ever), and I expect to iterate on this further.

If you want to see a larger example, check out the Snake source code. It could use some better formatting (and overall polish), but is already a big improvement over the old code that was hosted in Rust.