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

x/xnet/xnet_fuzz_test.go

@@ -283,6 +283,7 @@ func testStackSeeded(t *testing.T, seed1, seed2 int64) {
 		MTU:               mtu,
 		HardwareAddress:   [6]byte{0x1, 0, 0, 0, 0, v1},
 		AcceptMulticast:   v1%2 == 0,
+		PassivePeers:      1,
 	}
 	err := s1.Reset(cfg1)
 	if err != nil {
@@ -298,6 +299,7 @@ func testStackSeeded(t *testing.T, seed1, seed2 int64) {
 		MTU:               mtu,
 		HardwareAddress:   [6]byte{0x2, 0, 0, 0, 0, v2},
 		AcceptMulticast:   v2%2 == 0,
+		PassivePeers:      1,
 	}
 	err = s2.Reset(cfg2)
 	if err != nil {