feat: `SnapshotSummary` outline #2765

Job	Run time
build (ubuntu-latest)	1m 45s
build (macos-latest)	2m 14s
build (windows-latest)	7m 26s
build_with_no_default_features (ubuntu-latest)	29s
build_with_no_default_features (macos-latest)	38s
build_with_no_default_features (windows-latest)	3m 3s
check (ubuntu-latest)	3m 8s
check (macos-latest)	3m 27s
Verify MSRV	1m 46s
unit	8m 17s
	32m 13s

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