Minimize tokio dependence in async_std runtime and cleanup doc ambiguity #425

Job	Run time
docs	2m 3s
benches	3m 50s
Clippy	2m 2s
rustfmt	4s
Run the default tests	3m 48s
Test ractor in async_std	4m 26s
Test ractor with the `cluster` feature	3m 55s
Test ractor_cluster	3m 26s
	23m 34s

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