Access for "hexFromBase10" function

Hi,

I copied [hexFromBase10](https://github.com/pinarol/AuthentiCode/blob/8dbaaee728c9353e5de0730a4281aa0cc46351ee/Tests/AuthentiCodeTests/OCRATests.swift#L307) and have been using it and I thought you should know. You might consider moving that to main module and making it public. I know it's a small thing but it's useful. Thank you in advance.