A feedback-vertex-set is a set of vertices whose removal leaves a graph without cycles
(
https://en.wikipedia.org/wiki/Feedback_vertex_set).
We use this algorithm to spot the relevant places for adding
withdraw_gas statements in the
resulting Sierra code - there should be a
withdraw_gas call in every recursive call, or in
other words, in any cycle in the function call graph.
An efficient algorithm to find the minimum feedback-vertex-set in a directed graph is not known,
so here we implement some straight-forward algorithm that guarantees to cover all the cycles in
the graph, but doesn’t necessarily produce the minimum size of such a set.