Reimplementation of classes Ip4Address/Ip6Address/Ip4Prefix/Ip6Prefix
and the corresponding unit tests.

* Reimplemented classes Ip4Address and Ip6Address by inheriting from
  class IpAddress
* Reimplemented classes Ip4Prefix and Ip6Prefix by inheriting from
  class IpPrefix
* Reimplemented the unit tests Ip4AddressTest and Ip6AddressTest to
  match the corresponding IpAddressTest unit tests
* Reimplemented the unit tests Ip4PrefixTest and Ip6PrefixTest to
  match the corresponding IpPrefixTest unit tests
* Minor refactoring/cleanup of classes IpAddress and IpPrefix
12 files changed