* add ipv6 to xnet.StackAsync
* dns improvements
* improve DNS workings of StackAsync
* add tentative ICMPv6
* work on prefixes and fix some small bugs, plan UDP/TCP6
* fix bugs in StackAsync and ipv4.Prefix.Contains
* update arpsubtable
* completely remove legacy internet.StackIP for StackIPv4/v6
* ipv4/ipv6 tcp/udp
* add TCP6/UDP6 dialing APIs
* add xnet.Stack6 interface
* more ipv6 integration into StackAsync; various tweaks to lneto and documentation+TODOs
* add stack6 tests
* replace netip.Prefix with ipv4.Prefix where it makes sense
Pat Whittingslow