dag_walk: convert topo_order_* functions to async

[bnjmnt4n]

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[bnjmnt4n]

Due to the virality of async functions, converting dag_walk to async seems like it’s almost required at this point. I guess this is a major change, but I’m not sure if there are any other options (apart from maintaining a sync version with block_on if desired)?

[yuja]

Due to the virality of async functions, converting dag_walk to async seems like it’s almost required at this point. I guess this is a major change, but I’m not sure if there are any other options (apart from maintaining a sync version with block_on if desired)?

Maybe we have to use macro to strip off async/.await from blocking versions? It seems bad that we have to mark pure computation functions as async just because the underlying utility is unnecessarily async. Using .block_on() under the hood is equally bad.