The entity index reports an entity as alive the next frame, after a delete operation was pushed to the storage.

[Dandielo]

Describe the bug
Entity deletion from the ice::ecs::EntityIndex is happening after the worlds update methods are called. This results in an invalid state where the entity storage no longer has data for that entity (in the current implementation) however the entity index still returns true when checking if the entity is alive.

To Reproduce
After pushing an entity storage delete operation, the next frame will still report the entity as alive.

Expected behavior
After an entity is deleted from the storage it's deemed as destroyed and should be pushed on the 're-use' array.

[Dandielo]

Should be done along with #83. This is going to change after that issue is close.