IPAddress toString should use existing printTo algorithm and conform to RFC 5952 canonical format for IPv6 addresses

[sgryphon]

API component

IPAddress

Description

The toString() for an IPv6 type address has a different output from printTo(), and does not follow the RFC 5952 canonical format. It has an unnecessarily long fixed length format. Note that IPv4 does not use a fixed length format.

Also note that the existing printTo() method already implements the RFC 5952 canonical format algorithm, and has extensive tests (there are no tests for the toString() method).

The recommendation would be for toString() simply to use the existing algorithm from printTo() (and add some unit tests for toString).

Is this a breaking change?

No. Both formats are valid string formats for IPv6, and fromString() handles both of them (and all other valid formats), as should any other system. The canonical format is usually shorter, but will not exceed the fixed length format, so there is no issue with overruns.