hexBytes function
The bytes hex spells, two digits per byte.
Assumes an even number of hex digits, which is what a validated hex value holds; an odd count is a caller bug and throws where the last pair runs off the end.
Implementation
Uint8List hexBytes(String hex) => .fromList([
for (var offset = 0; offset < hex.length; offset += hexDigitsPerByte)
int.parse(hex.substring(offset, offset + hexDigitsPerByte), radix: hexRadix),
]);