Add ICMPv4 and lneto.StackNode and deprecate internet.StackNode (#65)

* begin adding icmp client

* rely on anon structs

* add tests

* tests passing

* icmp fleshed out

* rework icmp to include ip addr

* rename StackAsync.Demux/Encapsulate to RecvEthernet and SendEthernet

* remove legacy unreachable TCP tests

* remove uses of deprecated internet.StackNode in preference of lneto.StackNode

* documentation

* rename methods to signal no I/O happening

x/xnet/xnet_dns_test.go

@@ -49,7 +49,7 @@ func TestDNS_QueryReceivesAnswer(t *testing.T) {
 	// Client sends DNS query.
 	const carrierDataSize = MTU + ethernet.MaxOverheadSize
 	var buf [carrierDataSize]byte
-	n, err := client.Encapsulate(buf[:], -1, 0)
+	n, err := client.EgressEthernet(buf[:])
 	if err != nil {
 		t.Fatal("client Encapsulate failed:", err)
 	}
@@ -70,7 +70,7 @@ func TestDNS_QueryReceivesAnswer(t *testing.T) {
 	}
 
 	// Deliver response to client.
-	err = client.Demux(responsePkt, 0)
+	err = client.IngressEthernet(responsePkt)
 	if err != nil {
 		t.Fatal("client Demux failed:", err)
 	}