Add new() constructor.
Add convenience functions to convert addresses to/from human-readable
format. Store in class in binary representation; this makes ARP a
lot easier to implement (otherwise it would have to know about a dozen
converters).
Add convenience functions to convert addresses to/from human-
readable format. Store addresses in binary format during
construction or decoding (this makes it easier to validate the
input during construction rather than deferring until encoding).
Exercise constructor and decoder in testsuite.