Binary tree resolutions and fast sampling

[willdumm]

This PR includes:

• machinery to iterate through nodes which would be produced by resolving a multifurcation, and compute their supports, without explicitly choosing a single binary resolution (historydag.utils.iter_resolved_clade_supports)
• (for fun/testing) a function to produce complete DAGs containing all possible binary topologies on a set of leaf labels (historydag.dag.make_binary_complete_dag)
• a new general interface for passing data of sampled histories to an arbitrary target object without an intermediate history DAG representation (historydag.dag.PreorderTreeBuilder and subclasses)
• a faster, and non-recursive sampling method HistoryDag.fast_sample