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_concurrent_test.go

@@ -159,15 +159,15 @@ func kernelLoop(ctx context.Context, server *StackAsync, clients []StackAsync) {
 		}
 
 		// Process server outgoing -> route to appropriate client based on dest IP.
-		if n, _ := server.Encapsulate(buf, -1, 0); n > 0 {
+		if n, _ := server.EgressEthernet(buf); n > 0 {
 			routePacketToClient(buf[:n], clients)
 		}
 
 		// Process each client outgoing in randomized order.
 		rng.Shuffle(len(order), func(i, j int) { order[i], order[j] = order[j], order[i] })
 		for _, idx := range order {
-			if n, _ := clients[idx].Encapsulate(buf, -1, 0); n > 0 {
-				server.Demux(buf[:n], 0) // All clients talk to server.
+			if n, _ := clients[idx].EgressEthernet(buf); n > 0 {
+				server.IngressEthernet(buf[:n]) // All clients talk to server.
 			}
 		}
 
@@ -184,7 +184,7 @@ func routePacketToClient(pkt []byte, clients []StackAsync) {
 
 	for i := range clients {
 		if clients[i].Addr() == dstIP {
-			clients[i].Demux(pkt, 0)
+			clients[i].IngressEthernet(pkt)
 			return
 		}
 	}