* filtered each (apply a filter to the entities before to return them, it helps separating checks from logic)
* custom allocators and EnTT allocator-aware in general (long term feature, I don't actually need it at the moment) - see #22
* to analyze, long term feature: systems organizer based on dependency graphs for implicit parallelism (I don't want to think anymore in future :-))
* save/restore functionalities - see #27
* parent-child relationships between entities directly managed by the registry. is it possible to do that in a clean and safe way?
* blueprint registry - external tool, kind of factory to create entitites template for initialization
* scene management (I prefer the concept of spaces, that is a kind of scene anyway)
* raw view (affects sparse sets - custom iterator in derived one - and introduces a new kind of view): single component view to iterate components only instead of entities (in the right order!!)
  it should speed up systems like rendering or whatever requires a single component and isn't interested in the entity, for it avoids the double check of the get
* AOB