Rust crate

The rustdoc of treams-core is at https://yaugenst.github.io/treams-rs/latest/rust/treams_core/. treams-core is an internal crate: only the Python bindings in treams-py use it, and it is not published to crates.io. Its documentation includes private items, so it also serves as the reference for the numerical code. Build it locally with just docs-rust, which writes it to site/rust/.