TSort implements topological sorting using Tarjan's algorithm for strongly
connected components.

TSort is designed to be able to be used with any object which can be interpreted
as a directed graph.

TSort requires two methods to interpret an object as a graph, tsort_each_node
and tsort_each_child:
- tsort_each_node is used to iterate for all nodes over a graph.
- tsort_each_child is used to iterate for child nodes of a given node.

The equality of nodes are defined by eql? and hash since TSort uses Hash
internally.

WWW: https://github.com/ruby/tsort