D6: Cai–Ackerman–Freer showcase examples

[cameronfreer]

Sub-issue of the directed umbrella (#84); an attractive showcase PR after D3:

• undirected graphons as digraphons;
• tournament kernels;
• the unique exchangeable linear order and uniform finite orders;
• ordered-support construction for exchangeable DAGs;
• poset-compatible digraphons (exchangeable partial orders).

The di-IRM Bayesian block prior and inference machinery are deferred until good Dirichlet/stick-breaking infrastructure exists.