Add Erlang-inspired process group (`pg`) scopes #378

Job	Run time
benches	6m 52s
Clippy	3m 33s
docs	3m 2s
rustfmt	6s
Run the default tests	1m 29s
Test ractor in async_std	1m 27s
Test ractor with the `cluster` feature	8m 6s
Test ractor_cluster	5m 16s
	29m 51s

Provide feedback