Optimize scheduler

Profiling reveals https://github.com/dabreegster/abstreet/blob/master/sim/src/scheduler.rs as a hot spot, which makes sense. Anybody have ideas to speed it up? It's a priority queue with updateable priorities. There are lots of alternatives to std::collections::BinaryHeap, but I think I had trouble finding one that was deterministic (given the same inputs, simulations must run exactly the same).

I squeezed running a huge scenario from 71s to 63s using 7a0b9cd09a96e155a26f3304029d7c3ae753a9dc. I bet there's more possible.