Support class loader override #562

Job	Run time
rust_ci (macos-latest, stable)	1m 0s
rust_ci (macos-latest, beta)	54s
rust_ci (windows-latest, nightly)	3m 2s
rust_ci (windows-latest, beta)	1m 58s
rust_ci (ubuntu-latest, stable)	55s
rust_ci (windows-latest, stable)	3m 22s
rust_ci (ubuntu-latest, nightly)	1m 29s
rust_ci (ubuntu-latest, beta)	1m 1s
rust_ci (macos-latest, nightly)	1m 23s
	15m 4s

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