Coxeter diagram primitives for the RustUse geometry workspace.
`use-coxeter` stores nodes, labeled edges, and diagram counts. It does not implement Coxeter group
operations.
```rust
use use_coxeter::{CoxeterDiagram, CoxeterEdge, CoxeterNode};
let diagram = CoxeterDiagram::new(
).unwrap();
assert_eq!(diagram.node_count(), 2);
```