optional ahashmap

[Fogapod]

does it make sense to optionally depend on ahash or other hasher for better perf? there are only ~4-5 places with HashMap and HashSet. should be trivial to add a feature flag

[Michael-JB]

Thanks for the suggestion! Although I imagine adding ahash might improve performance slightly, it will add some complexity and so I'm a bit hesitant. I'll run some benchmarks to see how adding ahash affects things. My initial feeling is that there are probably lower-hanging performance improvements to make first (e.g., there are definitely some avoidable allocs at the moment).