all: use iterators over slice views more

This gets close to all of the remaining ones.

Updates #12912

Change-Id: I9c672bbed2654a6c5cab31e0cbece6c107d8c6fa
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/magicsock/debughttp.go

@@ -102,8 +102,7 @@ func (c *Conn) ServeHTTPDebug(w http.ResponseWriter, r *http.Request) {
 		sort.Slice(ent, func(i, j int) bool { return ent[i].pub.Less(ent[j].pub) })
 
 		peers := map[key.NodePublic]tailcfg.NodeView{}
-		for i := range c.peers.Len() {
-			p := c.peers.At(i)
+		for _, p := range c.peers.All() {
 			peers[p.Key()] = p
 		}