`pg`: Add `ScopeName` -> `Vec<GroupName>` mapping #420

Job	Run time
benches	3m 41s
rustfmt	10s
Clippy	2m 4s
docs	2m 6s
Run the default tests	3m 50s
Test ractor in async_std	3m 55s
Test ractor with the `cluster` feature	3m 53s
Test ractor_cluster	3m 27s
	23m 6s

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