passive MAC learning and ARP cache revamp (#96)

* begin working on arp cache

* fix little things

* rework arp cache priority

* fix test

* keep working on ARP

* push changes before thinking about ip prefix issue

* add passive peer MAC setting

* patch egress mac with correct ethernet CRCs

* consolidate subnet learning in subnetTable type

* add tests and fix subnet table bug

internet/stack-ethernet.go

@@ -39,6 +39,8 @@ type StackEthernet struct {
 	gwmac           [6]byte
 	mtu             uint16
 	acceptMulticast bool
+
+	onSend func(p []byte)
 	// crcupdate set when crc32 has been configured to be appended.
 	crcupdate func(crc uint32, p []byte) uint32
 }
@@ -63,6 +65,10 @@ func (ls *StackEthernet) HardwareAddr6() [6]byte {
 	return ls.mac
 }
 
+func (ls *StackEthernet) OnEncapsulate(cb func([]byte)) {
+	ls.onSend = cb
+}
+
 // Reset6 resets the stack with the given parameters.
 //
 // Deprecated: Use [StackEthernet.Configure] instead.
@@ -194,6 +200,9 @@ func (ls *StackEthernet) Encapsulate(carrierData []byte, offsetToIP, offsetToFra
 		dst[n] = 0
 		n++
 	}
+	if ls.onSend != nil {
+		ls.onSend(dst[:n])
+	}
 	if ls.crcupdate != nil {
 		crc := ls.crcupdate(0, carrierData[offsetToFrame:offsetToFrame+n])
 		binary.LittleEndian.PutUint32(carrierData[offsetToFrame+n:], crc)