Collect garbage on explicit gc call #484

Job	Run time
rust_ci (macos-latest, stable)	47s
rust_ci (macos-latest, beta)	52s
rust_ci (macos-latest, nightly)	55s
rust_ci (ubuntu-latest, stable)	53s
rust_ci (ubuntu-latest, beta)	52s
rust_ci (ubuntu-latest, nightly)	54s
rust_ci (windows-latest, stable)	2m 16s
rust_ci (windows-latest, beta)	1m 37s
rust_ci (windows-latest, nightly)	1m 47s
	10m 53s

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