util/set: add OfSliceView constructor and Set.AddSliceView

Building a Set from a views.Slice previously required set.Of(v.AsSlice()...),
which allocates an intermediate slice copy before allocating the set. Add
OfSliceView and AddSliceView to populate a set directly from the view,
mirroring the existing AddSlice/AddSeq/AddSet family.

Updates tailscale/corp#45499

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: I3f2a9d417c60be8e5f1acd42708e2f9a4d6c1b7e

util/set/set_test.go

@@ -7,6 +7,8 @@ import (
 	"encoding/json"
 	"slices"
 	"testing"
+
+	"tailscale.com/types/views"
 )
 
 func TestSet(t *testing.T) {
@@ -98,6 +100,18 @@ func TestSetOf(t *testing.T) {
 	}
 }
 
+func TestOfSliceView(t *testing.T) {
+	s := OfSliceView(views.SliceOf([]int{1, 2, 3, 4, 4, 1}))
+	if s.Len() != 4 {
+		t.Errorf("wrong len %d; want 4", s.Len())
+	}
+	for _, n := range []int{1, 2, 3, 4} {
+		if !s.Contains(n) {
+			t.Errorf("should contain %d", n)
+		}
+	}
+}
+
 func TestEqual(t *testing.T) {
 	type test struct {
 		name     string