Immer persist #398

Job	Run time
build-spm	25s
build	55s
check (Debug, gnu-6, 14)	1m 12s
docs	1m 6s
check (Debug, gnu-7, 14)	1m 3s
check (Debug, llvm-5, 14)	1m 10s
check (Debug, llvm-6, 14)	1m 22s
check (Release, gnu-6, 14)	1m 11s
check (Release, gnu-7, 14)	1m 6s
check (Release, llvm-5, 14)	1m 21s
check (Release, llvm-6, 14)	1m 21s
check (Debug, gnu-9, 14, coverage)	1m 21s
check (Debug, gnu-8, 17)	1m 9s
check (Debug, llvm-9, 17, fuzzers)	1m 21s
check (Debug, gnu-11, 20)	1m 10s
check (Debug, llvm-13, 20)	1m 21s
check (Debug, llvm-8, 14, sanitize)	1m 22s
check (Release, gnu-9, 14, benchmark)	1m 21s
check (Release, llvm-10, 14, benchmark)	1m 21s
	22m 38s

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