There are two kinds of entity maps:
- A primary map is used to allocate entity references and store the primary
entity data. This map only grows when adding new entities with 'push'.
- A secondary map contains additional information about entities in a primary
map. This map always grows with 'ensure', making default entries for any
unknown primary entities.
Only require the 'Default + Clone' traits for values stored in a secondary map.
Also remove the 'grow automatically' feature of the IndexMut implementation.
This means that clients need to call 'ensure' whenever using a potentially
unknown entity reference.
The 'grow automatically' feature could not be implemented for the Index trait,
and it seems unfortunate to have different semantics for Index and IndexMut.
