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

examples/httpserver/main.go

@@ -144,7 +144,7 @@ func run() (err error) {
 			return err
 		}
 		for {
-			nwrite, err := stack.Encapsulate(buf[:], -1, 0)
+			nwrite, err := stack.EgressEthernet(buf[:])
 			if err != nil {
 				log.Println("ERR:ENCAPSULATE", err)
 			} else if nwrite > 0 {
@@ -172,7 +172,7 @@ func run() (err error) {
 			if err != nil {
 				log.Fatal("goroutine read:", err)
 			} else if nread > 0 {
-				err = stack.Demux(buf[:nread], 0)
+				err = stack.IngressEthernet(buf[:nread])
 				if !errors.Is(err, lneto.ErrPacketDrop) {
 					err = logFrames("IN", buf[:nread])
 					if err != nil {