Revisit how the `EntityStorage` object is accessed and handled in the engine.

[Dandielo]

Currently the game cretates ice::ecs::EntityStorage objects and assigns them to worlds during world creation. This works as long each world has a different storage object, which is not what we are aiming for.

We want the ability to assign an entity storage to multiple worlds, and also the possibility to have multiple storage objects.

We should check if we can provide support for the following cases:

• [ ] An entity is allowed to have in each storage object a different archetype assigned.
• [ ] An entity is not allowed to have multiple archetypes assigned in the same storage.
• [ ] Each storage needs to have its own ice::ecs::ArchetypeIndex object.
• [ ] Each storage has an associated ice::ecs::EntityTracker object.
• [ ] Each storage has an associated ice::ecs::EntityOperations object.