`pg`: Add `ScopeName` -> `Vec<GroupName>` mapping #410

Job	Run time
Clippy	2m 4s
docs	2m 8s
benches	3m 53s
rustfmt	12s
Run the default tests	4m 3s
Test ractor in async_std	4m 4s
Test ractor with the `cluster` feature	4m 0s
Test ractor_cluster	3m 35s
	23m 59s

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