* 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)
* sparse_set: added optional user data to ::remove and ::clear
* sparse_set: remove the payload, it was really meh
* storage: review sigh_storage_mixin, simplified storage_adapter_mixin
* registry: pass itself twhen adding/patchin/removing components
test: updated tests for sparse set and poly storage
examples: added a test to avoid regressions on storage_adapter_mixin