Partial implementation of the ECS

[SmnTin]

Currently implemented features:

• Storage, MutStorage, Component and System C++20 concepts to conveniently verify classes conformance to the rules.
• A System can easily request arbitrary resource by simply listing a reference in the argument list of the update() method. It then automatically created and injected by the Dispatcher.
• Component's storage is a resource too! It can be easily accessed by possibly-const SomeComponent::Storage &.
• Components' storages can be efficiently joined via ecs::join() to iterate over entities that have all the specified components.
• While joining you can exclude entities that have certain component.
• And everything is implemented without a single virtual function using modern C++20 template programming.