Function saboten::cactusgraph::chain_edges

pub fn chain_edges<'a>(
    chain_pairs: &'a FnvHashSet<ChainPair>, 
    cactus_tree: &'a CactusTree<'a>
) -> FnvHashMap<(u64, u64), (u64, u64)>

Return the chain edges in the cactus tree as a map from pairs of net and chain vertices to chain pair snarls.