Add Display where it makes sense

[skeet70]

Places where we only have one basic implementation of Display, we should implement it. I was working with metadata results, and wanted to display the users a file was visible to. I expected to be able to:

metadata
  .visible_to_users()
  .join("\n")

but because Display wasn't implemented on something that is in reality VisibleUser(UserId(String)), I had to write:

metadata
  .visible_to_users()
  .iter()
  .map(|u| u.id().id())
  .collect::<Vec<_>>()
  .join("\n")