Implement garbage collection #478

Job	Run time
rust_ci (macos-latest, stable)	1m 0s
rust_ci (macos-latest, beta)	1m 2s
rust_ci (macos-latest, nightly)	1m 32s
rust_ci (ubuntu-latest, stable)	1m 0s
rust_ci (ubuntu-latest, beta)	1m 2s
rust_ci (ubuntu-latest, nightly)	1m 44s
rust_ci (windows-latest, stable)	2m 48s
rust_ci (windows-latest, beta)	1m 59s
rust_ci (windows-latest, nightly)	3m 16s
	15m 23s

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