Make persist compatible with C++17 #469

Job	Run time
build	19m 10s
build-spm	17s
docs	1m 11s
check (Debug, gnu-6, 14)	3m 6s
check (Debug, gnu-7, 14)	3m 4s
check (Debug, llvm-5, 14)	2m 46s
check (Debug, llvm-6, 14)	2m 45s
check (Release, gnu-6, 14)	3m 52s
check (Release, gnu-7, 14)	3m 52s
check (Release, llvm-5, 14)	3m 59s
check (Release, llvm-6, 14)	4m 12s
check (Debug, gnu-9, 14, coverage)	4m 6s
check (Debug, gnu-8, 17)	3m 14s
check (Debug, llvm-9, 17, fuzzers)	4m 12s
check (Debug, gnu-11, 20)	3m 43s
check (Debug, llvm-13, 20)	3m 7s
check (Debug, llvm-8, 14, sanitize)	4m 4s
check (Release, gnu-9, 14, benchmark)	18m 48s
check (Release, llvm-10, 14, benchmark)	19m 5s
	1h 48m 33s

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