TODO

* 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

EXAMPLES
* filter on runtime values/variables (not only types)
* support to polymorphic types (see #859)

DOC:
* storage<void>
* custom storage/view
* examples (and credits) from @alanjfs :)

WIP:
* add storage getter for filters to views and groups
* exploit the tombstone mechanism to allow enabling/disabling entities (see bump, compact and clear for further details)
* emitter: runtime handlers, allocator support (ready for both already)
* view: no storage_traits dependency -> use storage instead of components for the definition
* basic_storage::bind for cross-registry setups (see and remove todo from entity_copy.cpp)
* uses-allocator construction: any (with allocator support), poly, ...
* process scheduler: reviews, use free lists internally
* iterator based try_emplace vs try_insert for perf reasons
* dedicated entity storage, in-place O(1) release/destroy for non-orphaned entities, out-of-sync model
* entity-only and exclude-only views (both solved with entity storage and storage<void>)
* custom allocators all over (registry, ...)
* add test for maximum number of entities reached
* add user data to type_info
* make runtime views use opaque storage and therefore return also elements.
* 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