Implement garbage collection #479

Job	Run time
rust_ci (macos-latest, stable)	47s
rust_ci (macos-latest, beta)	52s
rust_ci (macos-latest, nightly)	54s
rust_ci (ubuntu-latest, stable)	48s
rust_ci (ubuntu-latest, beta)	57s
rust_ci (ubuntu-latest, nightly)	51s
rust_ci (windows-latest, stable)	2m 49s
rust_ci (windows-latest, beta)	1m 40s
rust_ci (windows-latest, nightly)	2m 4s
	11m 42s

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