Props allocator and possibility for storing props and state in user space ECS

[PsichiX]

Good news, i've figured out general design of Props Allocator which will allow us to optimize storage and access to props and state in RAUI, also it will allow us to make allocators that will use userspace ECS (specs, bevy_ecs, etc) as a storage.
During next days i'll describe more and more design details.