Add Erlang-inspired process group (`pg`) scopes #382

Job	Run time
benches	6m 43s
Clippy	3m 9s
docs	4m 14s
rustfmt	6s
Run the default tests	1m 16s
Test ractor in async_std	1m 14s
Test ractor with the `cluster` feature	1m 14s
Test ractor_cluster	4m 38s
	22m 34s

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