TODO

* 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
* allow to replace std:: with custom implementations
* add examples (and credits) from @alanjfs :)
* custom pools example (multi instance, tables, enable/disable, and so on...)

WIP:
* remove view/storage dispatcher, add support to relax policy constraints on user request (eg view.use<T>())
* improve perf for sparse_set/storage::insert/emplace/destroy/remove/...
* custom allocators all over

WIP:
* make value_type available from meta container types, otherwise we have to default construct a container to get it
* make it possible to register externally managed pools with the registry (allow for system centric mode)
* registry: switch to the udata/mixin model and get rid of poly storage, use pointer to sparse set only for pools, discard pool_data type.
* it's now possible to have 0 as null entity/version, so we can finally switch to it
* make pools available (registry/view/group), review operator| for views
* page size: add per-pool size, allow for 0 sizes (old fully packed array)
* compressed pair to exploit ebo in sparse set and the others
* isolate view iterator, unwrap iterators in registry ::remove/::erase/::destroy to use the faster solution for non-view iterators
* remove view each<T>(F), each<T>(), make view::use return a view and remove the mutable data member
* 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
* any_vector for context variables
* make const registry::view thread safe, switch to a view<T...>{registry} model (long term goal)
* weak reference wrapper example with custom storage
* 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
* add example: 64 bit ids with 32 bits reserved for users' purposes