Provide descendant() and anscestors() functions

[voxoid0]

The library really should provide descendants() and ancestors() functions (recursive versions of predecessors() and successors()) out of the box, like python's networkx lib does. This was mentioned before in issue #34 .

[cogell]

Hey mate, I saw that the concern in issue #34 was the recursive versions hit stack-overflow issues in JS while imperative versions didn't.

Why do you want descendants() and ancestors() despite this? What am I missing? 😅