Replace core language HexStrings with library entity

[Walter Bright via Digitalmars-d]

On 3/15/2015 4:28 PM, Andrei Alexandrescu wrote:
> That's what nice about library artifacts vs. language artifacts: you can add
> more of the former! Define hexBytes and hexString. -- Andrei

Yup. Should resist the urge to "fix" hexString, just make it a dropin replacement.