+ Make parse_radix terser and clearer.
+ Make purpose of radix clearer at use site
(note that the code currently completely misuses the --address-radix
flag; this is inherited from the previous code)
+ Don't panic! inside parse_radix; instead return Result<> and let the
caller handle errors (currently we panic, but probably we'll want to use
some less alarming error routine); this will be more testable later as
well.
