Avoid method in method

And rename `File.concat_path` to `File._concat_path`.

mrbgems/mruby-io/mrblib/file.rb

@@ -55,8 +55,7 @@ class File < IO
     s
   end
 
-  def self.expand_path(path, default_dir = '.')
-    def concat_path(path, base_path)
+  def self._concat_path(path, base_path)
     if path[0] == "/" || path[1] == ':' # Windows root!
       expanded_path = path
     elsif path[0] == "~"
@@ -87,14 +86,15 @@ class File < IO
         expanded_path += "/"
       end
     else
-        expanded_path = concat_path(base_path, _getwd)
+      expanded_path = _concat_path(base_path, _getwd)
       expanded_path += "/" + path
     end
 
     expanded_path
   end
 
-    expanded_path = concat_path(path, default_dir)
+  def self.expand_path(path, default_dir = '.')
+    expanded_path = _concat_path(path, default_dir)
     drive_prefix = ""
     if File::ALT_SEPARATOR && expanded_path.size > 2 &&
         ("A".."Z").include?(expanded_path[0].upcase) && expanded_path[1] == ":"