Add Display where it makes sense

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:

```rs
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:

```rs
metadata
.visible_to_users()
.iter()
.map(|u| u.id().id())
.collect::>()
.join("\n")
```