fails:String#hex treats leading characters of self as a string of hex digits
fails:String#hex does not accept a sequence of underscores as part of a number
fails:String#hex takes an optional sign
fails:String#hex takes an optional 0x
fails:String#hex requires that the sign is in front of the 0x if present
fails:String#hex returns 0 on error
fails:String#hex returns 0 if sequence begins with underscore
