walk: add a walk function

walk.go

+package files
+
+import (
+	"path/filepath"
+)
+
+// Walk walks a file tree, like `os.Walk`.
+func Walk(nd Node, cb func(fpath string, nd Node) error) error {
+	var helper func(string, Node) error
+	helper = func(path string, nd Node) error {
+		if err := cb(path, nd); err != nil {
+			return err
+		}
+		dir, ok := nd.(Directory)
+		if !ok {
+			return nil
+		}
+		iter := dir.Entries()
+		for iter.Next() {
+			if err := helper(filepath.Join(path, iter.Name()), iter.Node()); err != nil {
+				return err
+			}
+		}
+		return iter.Err()
+	}
+	return helper("", nd)
+}