* 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:
* add the possibility of disabling entities without deleting components thanks to the new full check
* add registry::storage id -> basic_sparse_set (no template arg), add example of use
* fast-contains for sparse sets (low prio but nice-to-have)
* runtime events (dispatcher/emitter), runtime context variables...
* runtime_view/registry, remove reference to basic_sparse_set<E>
* 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
* resource, forward the id to the loader from the cache and if constexpr the call to load, update doc and describe customization points
* add user data to type_info
* 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