Assorted algorithms.

spt_closure: Implementation and proof of correctness of a reachability/closure operation over num_set num_maps.

topological_sort: A topological-sort-like partitioning algorithm, using sptrees under-the-hood and relying on spt_closure. Strictly speaking this is not a topological sort because it allow cycles in the dependencies. The implementation is a divide-and-conquer algorithm.