Dataflow construction seems slow-ish

I'm not certain this is a real issue, but the event_driven example can be made to create 1000 dataflows each a sequence of 1000 nodes, and doing so takes about 10s on my modern desktop. That seems like a long time to put together what is meant to be a linear amount of information. It does seem to be linear (scaling up either number by 10 changes the time by 10), but it would still be good to get a sense for what is going on. Initial investigation suggests allocations in reachability, which indeed does have a bunch of small per-operator allocations. Perhaps those could be consolidated somehow?

Reproduce the reported case using the event_driven example: 1000 dataflows of 1000 nodes, and measure construction time. Then inspect the reachability path and its per-operator allocations using profiling or allocation measurements. Done means explaining the bottleneck and demonstrating a measured improvement or confirming that no actionable change is warranted.