Collect class instances to be used in gc #474

Job	Run time
rust_ci (macos-latest, stable)	1m 21s
rust_ci (macos-latest, beta)	2m 0s
rust_ci (macos-latest, nightly)	1m 21s
rust_ci (ubuntu-latest, stable)	1m 47s
rust_ci (ubuntu-latest, beta)	1m 35s
rust_ci (ubuntu-latest, nightly)	1m 36s
rust_ci (windows-latest, stable)	4m 27s
rust_ci (windows-latest, beta)	3m 40s
rust_ci (windows-latest, nightly)	2m 53s
	20m 40s

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