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

errors.go

@@ -20,6 +20,13 @@ const (
 	ErrInvalidField                         // invalid field
 	ErrInvalidLengthField                   // invalid length field
 	ErrExhausted                            // resource exhausted
+	ErrAlreadyRegistered                    // protocol already registered
+
+	// Below are potentially good future error additions
+	// based on one or two encountered use cases, example use case included.
+	/*
+		- ErrUnregistered/ErrAborted // connection unregistered. i.e: ICMP client aborted during active ping, ping process returns this.
+	*/
 )
 
 func (err errGeneric) Error() string {