* long term feature: shared_ptr less locator and resource cache
* custom allocators and EnTT allocator-aware in general (long term feature, I don't actually need it at the moment) - see #22
* 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 :)
* static reflection, hint: template<> meta_type_t<Type>: meta_descriptor<name, func..., props..., etc...> (see #342)
* update documentation for meta, it contains less than half of the actual feature

* custom pools example:
  - lockless fully concurrent ro/rw pool with free lists
  - multi instance
  - tables
  - enable/disable component
  - spatial query
  - runtime types pool
  - off-line/off-memory/remote
  - poly
  - ...

WIP:
* HP: headless (sparse set only) view
* HP: pass the registry to pools, basic poly storage should have only component member
* HP: make view pack work also with groups, make packs input iterator only, add view adapter for external sources
* HP: write documentation for custom storages and views!!
* HP: add support for const references to any/poly (actual copy on copy, data vs cdata)
* HP: any/poly: configurable sbo size, compile-time policies like sbo-required.
* HP: registry: use a poly object for pools, no more pool_data type.
* HP: make runtime views use opaque storage and therefore return also elements.
* suppress warnings in meta.hpp (uninitialized members)
* add exclude-only views to combine with packs
* deprecate non-owning groups in favor of owning views and view packs, introduce lazy owning views
* view pack: plain function as an alias for operator|, reverse iterators, rbegin and rend
* pagination doesn't work nicely across boundaries probably, give it a look. RO operations are fine, adding components maybe not.
* add observer functions aside observer class
* snapshot: support for range-based archives
* update snapshot documentation to describe alternatives
* page size 0 -> page less mode
* add example: 64 bit ids with 32 bits reserved for users' purposes
* add meta dynamic cast (search base for T in parent, we have the meta type already)
* make meta base/conv node work with storage/any and deprecate/remove meta_base, meta_conv, ...