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>

util/set/slice.go

@@ -67,7 +67,7 @@ func (ss *Slice[T]) Add(vs ...T) {
 
 // AddSlice adds all elements in vs to the set.
 func (ss *Slice[T]) AddSlice(vs views.Slice[T]) {
-	for i := range vs.Len() {
-		ss.Add(vs.At(i))
+	for _, v := range vs.All() {
+		ss.Add(v)
 	}
 }