net/flowtrack: add Tuple.String method

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/flowtrack/flowtrack.go

@@ -12,6 +12,7 @@ package flowtrack
 
 import (
 	"container/list"
+	"fmt"
 
 	"inet.af/netaddr"
 )
@@ -22,6 +23,10 @@ type Tuple struct {
 	Dst netaddr.IPPort
 }
 
+func (t Tuple) String() string {
+	return fmt.Sprintf("(%v => %v)", t.Src, t.Dst)
+}
+
 // Cache is an LRU cache keyed by Tuple.
 //
 // The zero value is valid to use.