* long term feature: shared_ptr less locator and resource cache
* debugging tools (#60): the issue online already contains interesting tips on this, look at it
* work stealing job system (see #100) + mt scheduler based on const awareness for types
* add examples (and credits) from @alanjfs :)

WIP:
* fast-contains for sparse sets (low prio but nice-to-have)
* runtime components (registry), runtime events (dispatcher/emitter), runtime context variables ...
* runtime_view/registry, remove reference to basic_sparse_set<E>
* make pools available (registry/view/group), review operator| for views, make views accept registry to ctor
* make view.lead() or similar available to return leading pool (useful for mt)
* dedicated entity storage, in-place O(1) release/destroy for non-orphaned entities, out-of-sync model
* custom allocators all over

WIP:
* customizable any_vtable, sfinae-friendly definition and op::custom for user-def
* registry: fake vtable (mixin injected) to get rid of poly storage, use pointer to sparse set only for pools, discard pool_data type.
* general purpose paged vector container
* resource, forward the id to the loader from the cache and if constexpr the call to load, update doc and describe customization points
* make it possible to create views of the type `view<T, T>`, add get by index and such, allow to register custom pools by name with the registry
* add user data to type_info
* make const registry::view thread safe, switch to a view<T...>{registry} model (long term goal)
* headless (sparse set only) view
* write documentation for custom storages and views!!
* make runtime views use opaque storage and therefore return also elements.
* add exclude-only views to combine with packs
* entity-aware observer, add observer functions aside observer class
* deprecate non-owning groups in favor of owning views and view packs, introduce lazy owning views
* snapshot: support for range-based archives