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

DOC:
* custom storage/view
* examples (and credits) from @alanjfs :)
* update entity doc when the storage based model is in place
* in-place O(1) release/destroy for non-orphaned entities, out-of-sync model

TODO (high prio):
* check natvis files (periodically :)
* resource cache: avoid using shared ptr with loader and the others
* further optimize exclusion lists in multi type views (no existence check)
* doc: bump entities
* update view doc: single vs multi type views are no longer a thing actually
* further improve meta resolve function by id (bimap)
* storage-based invoke helpers (update sigh mixin to also support storage based cbs)
* make nth_argument use function types rather than pointers
* use fixed_size_sequence_container_v to skip calls in meta.hpp, make funcs ret type void
* review storage entity value_type and type passed to sparse set
* storage entity tests for sort and sort_as (instead of pack)
* ===> TEST: view (scoped begin) tests after the last changes

WIP:
* get rid of observers, storage based views made them pointless - document alternatives
* exploit the tombstone mechanism to allow enabling/disabling entities (see bump, compact and clear for further details)
* process scheduler: reviews, use free lists internally
* deprecate non-owning groups in favor of owning views and view packs, introduce lazy owning views
* bring nested groups back in place (see bd34e7f)
* work stealing job system (see #100) + mt scheduler based on const awareness for types