0.3.0
=====
* Switched to `optional` crate, remove invalid_value module.
0.2.0
=====
* Added debug-only check that `ObjId` used to get object from `ObjPool` is got from the correct `ObjPool`.
* Reintroduced iterators.
* Added `OptionObjId`.
0.1.0
=====
Initial version.
* Undeprecated `vec-arena`.
* Added `ObjId`.