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

internal/slices.go

@@ -79,3 +79,11 @@ func BytesEqual(a, b []byte) bool {
 	}
 	return unsafe.String(&a[0], len(a)) == unsafe.String(&b[0], len(b))
 }
+
+func SliceDequeueFront[T any](a *[]T) T {
+	s := *a
+	v := s[0]
+	n := copy(s, s[1:])
+	*a = s[:n]
+	return v
+}