add ipv6 to xnet.StackAsync (#107)

* 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

dhcpv4/dhcp_test.go

@@ -1,10 +1,10 @@
 package dhcpv4
 
 import (
-	"net/netip"
 	"testing"
 
 	"github.com/soypat/lneto/internal"
+	"github.com/soypat/lneto/ipv4"
 )
 
 func TestClientServer(t *testing.T) {
@@ -29,7 +29,7 @@ func TestClientServer(t *testing.T) {
 	}
 	sv.Configure(ServerConfig{
 		ServerAddr: svAddr,
-		Subnet:     netip.PrefixFrom(netip.AddrFrom4(svAddr), 24),
+		Subnet:     ipv4.PrefixFrom(svAddr, 24),
 	})
 	// CLIENT DISCOVER.
 	assertClState(StateInit)