code: func NewHashingFs(delegate afero.Fs, hashReceiver FileHashReceiver) afero.Fs {
  	return &md5HashingFs{Fs: delegate, hashReceiver: hashReceiver}
  }
docs: // NewHashingFs creates a new filesystem that will receive MD5 checksums of
  // any written file content on Close. Note that this is probably not a good
  // idea for "full build" situations, but when doing fast render mode, the amount
  // of files published is low, and it would be really nice to know exactly which
  // of these files where actually changed.
  // Note that this will only work for file operations that use the io.Writer
  // to write content to file, but that is fine for the "publish content" use case.