A Rust implementation of the Cassowary linear constraint solving algorithm. This is a fork of `cassowary-rs` originally written by Dylan Ede. It makes a number of improvements including: * `Solver`s are Send + Sync * new variable types can be defined, removing the need to maintain a lookup map The Cassowary algorithm is designed for naturally laying out user interfaces using linear constraints, like 'this button must line up with this text box'.