Accessing allObjects from DiskStorage

I've noticed an issue with the allObjects property in the DiskStorage implementation. The current default implementation returns an empty array, making it impossible to access any stored objects. Also, as of now there is no way to get allObjects from disk when they are not in memory?

The relevant code snippet from DiskStorage is as follows:

```
extension DiskStorage: StorageAware {
public var allKeys: [Key] { [] }

public var allObjects: [Value] { [] }
}
```

Start at the DiskStorage: StorageAware extension shown in the issue and trace how objects are persisted on disk. Determine the intended behavior for allObjects when values are not in memory, then add or update tests covering retrieval of all stored objects.

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