This change adds extension to the CIDR and IPAddress types to implement
custom encode/decode functions for `Codable` conformance to use their
string representation as the output from encode and input to decode.
This would make the output from encoding this type (e.g. JSON) more
human-readable rather than using the internal integer representation.
These types can benefit from swift's automatic
synthesis as the properties already conform to Codable. This will give
some flexibility for clients using these types to not have to add
extension and implement the encoding / decoding separately.
Kathryn Baldauf