refactor: make `hex_as_bytes` DRY #4676

Job	Run time
ci (ubuntu-20.04, gcc, g++, 20, Release)	8m 9s
ci (ubuntu-20.04, gcc, g++, 20, Debug)	7m 26s
ci (ubuntu-20.04, clang, clang++, 20, Release)	8m 3s
ci (ubuntu-20.04, clang, clang++, 20, Debug)	7m 4s
ci (ubuntu-22.04, gcc, g++, 20, Release)	7m 17s
ci (ubuntu-22.04, gcc, g++, 20, Debug)	6m 27s
ci (ubuntu-22.04, clang, clang++, 20, Release)	8m 48s
ci (ubuntu-22.04, clang, clang++, 20, Debug)	8m 6s
ci (macos-12, gcc, g++, 20, Release)	8m 12s
ci (macos-12, gcc, g++, 20, Debug)	7m 1s
ci (macos-14, clang, clang++, 20, Release)	4m 5s
ci (macos-14, clang, clang++, 20, Debug)	3m 42s
ci (windows-2022, cl, cl, 20, Release)	7m 21s
ci (windows-2022, cl, cl, 20, Debug)	6m 25s
	1h 38m 6s

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