Minimize tokio dependence in async_std runtime and cleanup doc ambiguity #423

Job	Run time
Clippy	2m 11s
benches	3m 58s
docs	2m 4s
rustfmt	8s
Run the default tests	3m 54s
Test ractor in async_std	3m 56s
Test ractor with the `cluster` feature	3m 49s
Test ractor_cluster	3m 34s
	23m 34s

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