entt/TODO

* long term feature: templated generic vm
* long term feature: shared_ptr less locator
* long term feature: shared_ptr less 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
* runner proposal: https://en.wikipedia.org/wiki/Fork%E2%80%93join_model https://slide-rs.github.io/specs/03_dispatcher.html
* work stealing job system (see #100)
* meta: sort of meta view based on meta stuff to iterate entities, void * and meta info objects
* allow for built-in parallel each if possible
* allow to replace std:: with custom implementations
* remove runtime views, welcome reflection and what about snapshot?
* empty components model allows for shared components and prefabs unity-like
  - each with entity return the shared component multiple times, one per entity that refers to it
  - each components only return actual component, so shared components are returned only once
* types defined at runtime that refer to the same compile-time type (but to different pools) are possible, the library is almost there
* add take functionality, eg registry.take(entity, other); where it takes the entity and all its components from registry and move them to other
* add opaque input iterators to views and groups that return tuples <entity, T &...> (proxy), multi-pass guaranteed
* add fast lane for raw iterations, extend mt doc to describe allowed add/remove with pre-allocations on fast lanes
* review sparse set to allow customization (mix pack in the spec, base is position only)
  - non-owning groups can iterate pages and skip empty ones, this should mitigate the lack of the packed array
* review 64 bit id: user defined area + dedicated member on the registry to set it
* early out in views using bitmasks with bloom filter like access based on modulus
  - standard each, use bitmask to speed up the whole thing and avoid accessing the pools to test for the page
  - iterator based each with a couple of iterators passed from outside (use bitmask + has)
* stable component handle that isn't affected by reallocations
* multi component registry::remove and some others?
* move CONTRIBUTING.md within doc (for GitHub)
* reactive systems