feat: switch to a single routability event

This means we can make the event _stateful_ so subscribing always gives us the
last state.

event/routability.go

 package event
 
-// EvtLocalRoutabilityPrivate is an event struct to be emitted with the local's
-// node routability changes to PRIVATE (i.e. not routable from the Internet).
-//
-// This event is usually emitted by the AutoNAT subsystem.
-type EvtLocalRoutabilityPrivate struct{}
-
-// EvtLocalRoutabilityPublic is an event struct to be emitted with the local's
-// node routability changes to PUBLIC (i.e. appear to routable from the
-// Internet).
-//
-// This event is usually emitted by the AutoNAT subsystem.
-type EvtLocalRoutabilityPublic struct{}
+import (
+	"github.com/libp2p/go-libp2p-core/network"
+)
 
-// EvtLocalRoutabilityUnknown is an event struct to be emitted with the local's
-// node routability changes to UNKNOWN (i.e. we were unable to make a
-// determination about our NAT status with enough confidence).
+// EvtLocalRoutability is an event struct to be emitted with the local's node
+// routability changes state.
 //
 // This event is usually emitted by the AutoNAT subsystem.
-type EvtLocalRoutabilityUnknown struct{}
+type EvtLocalRoutability struct {
+	Routability network.Routability
+}

network/network.go

@@ -53,6 +53,24 @@ const (
 	CannotConnect
 )
 
+// Routability indicates how reachable a node is.
+type Routability int
+
+const (
+	// RoutabilityUnknown indicates that the routability status is unknown.
+	RoutabilityUnknown = iota
+
+	// RoutabilityPublic indicates that the node is reachable from the
+	// public internet.
+	RoutabilityPublic
+
+	// RoutabilityPrivate indicates that the node is not reachable from the
+	// public internet.
+	//
+	// NOTE: This node may _still_ be reachable via relays.
+	RoutabilityPrivate
+)
+
 // Stat stores metadata pertaining to a given Stream/Conn.
 type Stat struct {
 	Direction Direction