add 'Prefix' object and some more helper routines

set.go

@@ -26,7 +26,7 @@ func (s *Set) Len() int {
 }
 
 func (s *Set) Keys() []*Cid {
-	var out []*Cid
+	out := make([]*Cid, 0, len(s.set))
 	for k, _ := range s.set {
 		c, _ := Cast([]byte(k))
 		out = append(out, c)
@@ -42,3 +42,14 @@ func (s *Set) Visit(c *Cid) bool {
 
 	return false
 }
+
+func (s *Set) ForEach(f func(c *Cid) error) error {
+	for cs, _ := range s.set {
+		c, _ := Cast([]byte(cs))
+		err := f(c)
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}