wrote HexToDec & test

I'd like to add some math to this as well, or move it into calculator.pm (or at least consolidate the HexTo* plugins), but here's a start, and I'll use this all the time as is. Hopefully some others will too :-).
I think `bignum` overrides `hex()` but I don't think `Math::BigInt` does. Not sure if that's a problem.