entt/TODO

* long term feature: shared_ptr less locator
* long term feature: shared_ptr-less resource cache
* custom allocators and EnTT allocator-aware in general (long term feature, I don't actually need it at the moment) - see #22
* scene management (I prefer the concept of spaces, that is a kind of scene anyway)
* debugging tools (#60): the issue online already contains interesting tips on this, look at it
* define basic reactive systems (track entities to which component is attached, track entities from which component is removed, and so on)
* define systems as composable mixins (initializazion, reactive, update, whatever) with flexible auto-detected arguments (registry, views, etc)
* registry::create with a "hint" on the entity identifier to use, it should ease combining multiple registries
* optimize for empty components, it would be a mid improvement in terms of memory usage (see std::is_empty)
* deep copy of a registry (or use the snapshot stuff to copy components and keep intact ids at least)
* is it possible to iterate all the components assigned to an entity through a common base class?
* prototype should offer a way to retrieve its internal registry
* can we do more for shared libraries? who knows... see #144
* work stealing job system (see #100)
* composable looper so as to pack erased systems, compose runners at different rates and run them at once in the loop
* meta: sort of meta view based on meta stuff to iterate entities, void * and meta info objects
* registry::probe<component>(entt) (returns a component * if entt has the component, nullptr otherwise)
* hashed string: add implicit check on construction for uniqueness (optional)
* add a note about multithreading support to the README file
* signals on entity creation/destruction
* flexible views with "composable" filters
* do not set BUILD_TESTING to ON by default anymore
* create a dedicated .md for EnTT in Action list and create two sections: articles and games