Remove dependency on slab #3253

Job	Run time
test (1.26)	6m 56s
test (stable)	3m 32s
test (1.30)	4m 56s
test (beta)	3m 21s
test (no rustup)	1m 57s
test (1.31)	5m 3s
test (nightly)	3m 18s
test (1.36)	6m 20s
test (nightly, macos-latest)	3m 38s
test (1.39)	6m 7s
test (nightly, windows-latest)	4m 47s
test (1.41)	5m 48s
msrv / msrv	4m 33s
deny / deny	14s
release-dry-run / prepare	3s
miri / miri	1m 3s
tidy / tidy	1m 15s
release-dry-run / build (aarch64-unknown-linux-gnu)	1m 32s
release-dry-run / build (aarch64-unknown-linux-musl)	1m 5s
release-dry-run / build (aarch64-apple-darwin, macos-12)	1m 17s
release-dry-run / build (aarch64-pc-windows-msvc, windows-2019)	2m 45s
release-dry-run / build (x86_64-unknown-linux-gnu)	46s
release-dry-run / build (x86_64-unknown-linux-musl)	54s
release-dry-run / build (x86_64-apple-darwin, macos-12)	1m 14s
release-dry-run / build (x86_64-pc-windows-msvc, windows-2019)	3m 2s
release-dry-run / build (x86_64-unknown-freebsd)	1m 12s
release-dry-run / build (x86_64-unknown-illumos)	45s
release-dry-run / build (universal-apple-darwin, macos-12)	1m 26s
	1h 18m 49s

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