* drop utility.hpp
* turn get_t/exclude_t into aliases for type_list
* updated api for basic_registry<...>::group/group_if_exists
* doc review to reflect all the changes above
* added ::iterate and ::exclude member functions to attach pools at runtime
* removed vector-based constructor
* removed basic_registry<...>::runtime_view member function
* updated benchmarks accordingly (also reviewed them as a whole just because)
* remove visit
* added storage_proxy_iterator (input iterator category, modeled as a random access iterator)
* added ::storage() (const and non-const) to return an iterable object over all pools
* added page_size to tune the page size on a per-type basis
* turned in_place_delete into a boolean value
* turned ignore_if_empty into a boolean value
* removed in_place_delete_v (no longer required)
* added basic_sparse_set<E>::current to return known version of identifiers in set
* sparse arrays also contain updated entity versions, no more unused bits
* basic_sparse_set<E>::contains performs a strict version check
* sparse sets manage correctly foreign entities in all cases now
* no tombstone checks for multi-type views, zero-cost pointer stability model
* review of entt_traits design
* added static constexpr member function entt_traits::to_integral
* added static constexpr member function entt_traits::to_entity
* added static constexpr member function entt_traits::to_version
* added static constexpr member function entt_traits::to_type
* custom class identifiers must expose member type entity_field
* it's no longer required to specialize entt_traits (breaking change)
* allocator support for sparse set and storage
* pointer stability when assigning components
* removed ::raw functions (registry, view, group)
* to_entity returns null for invalid components
