refactor: make `hex_as_bytes` DRY #4663

Job	Run time
ci (ubuntu-20.04, gcc, g++, 20, Release)	8m 1s
ci (ubuntu-20.04, gcc, g++, 20, Debug)	7m 15s
ci (ubuntu-20.04, clang, clang++, 20, Release)	8m 11s
ci (ubuntu-20.04, clang, clang++, 20, Debug)	7m 9s
ci (ubuntu-22.04, gcc, g++, 20, Release)	7m 4s
ci (ubuntu-22.04, gcc, g++, 20, Debug)	6m 39s
ci (ubuntu-22.04, clang, clang++, 20, Release)	9m 1s
ci (ubuntu-22.04, clang, clang++, 20, Debug)	7m 46s
ci (macos-12, gcc, g++, 20, Release)	7m 34s
ci (macos-12, gcc, g++, 20, Debug)	7m 42s
ci (macos-14, clang, clang++, 20, Release)	5m 27s
ci (macos-14, clang, clang++, 20, Debug)	10m 25s
ci (windows-2022, cl, cl, 20, Release)	7m 30s
ci (windows-2022, cl, cl, 20, Debug)	6m 26s
	1h 46m 10s

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