std0

Fix MemMapFs.Readdir to use files buffer

2

To match behavior of `os.File.Readdir`, `MemMapFs.Readdir` should use files buffer instead of retrieving currently stored files on each call. Fixes #261

`os.File.Readdir` under the hood uses buffering of directory contents, which gives it ability to traverse directory and remove files at the same time. Currently, `MemMapFs.Readdir` retrieves files from `f.fileData.memDir.Files()` on...