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

internet/definitions_go.go

@@ -2,7 +2,9 @@
 
 package internet
 
-func makecbnode(s StackNode) cbnode {
+import "github.com/soypat/lneto"
+
+func makecbnode(s lneto.StackNode) cbnode {
 	return cbnode{
 		_s: s,
 	}
@@ -10,7 +12,7 @@ func makecbnode(s StackNode) cbnode {
 
 type cbnode struct {
 	// Do not access outside of handlers/node logic.
-	_s StackNode
+	_s lneto.StackNode
 }
 
 func (s cbnode) Encapsulate(carrierData []byte, offsetToIP, offsetToFrame int) (int, error) {