v0.4.0 unsorted mode

[djthread]

- `fields` list was optional, but the behavior when none declared has changed.
  * Before, the `id_key` (default `:id`) was automatically used in the
    background. This meant that presorted indexes (ascending and descending)
    were auto-maintained and used as the default when calling `get_records` etc. 
  * Now, no such presorted indexes will be maintained. When querying,
    `:ets.tab_to_list/2` will be used, with no particular order being promised.