Extending Icmpv6Type

[xyzzyz]

Currently, Icmpv6Type is an enum with a bunch of variants, and a catch-all Unknown variant. Since it's not marked as non_exhaustive, adding new variants would break backwards compatibility. At the same time, it would be highly desirable to be able to add new variants, e.g. to cleanly support MLDv1 and MLDv2, which are subprotocols of ICMPv6.

What are your thoughts on breaking backwards compatibility? Since etherparse is still on 0.*, I think it would be legal from sem-ver perspective. It would be somewhat annoying to users, but it wouldn't be that hard to update the code; users should already have a match case to handle Unknown case, so they'd just need to adjust the pattern to capture both unknown, and also unsupported variants.