The new data structure uses open addressing instead of chaining to resolve collisions in the hash table.
This new structure was never slower than the old implementation in my tests. Code that first inserts all edges and then iterates through all edges (e.g. to remove duplicates) benefits the most, because the `EdgeHashIterator` becomes a simple for loop over a continuous array.
Reviewer: campbellbarton
Differential Revision: D4050
aa63a87d37