EntityManager class
Manages the entities for the user.
Manages the creation, and destruction of entity IDs for the user.
Public functions
- auto CreateEntity() -> EntityID
- Creates an entity, returning a free one when possible.
- void DestroyEntity(EntityID entity)
- Marks the given entity as 'free'.
- auto IsAlive(EntityID entity) const -> bool
- Returns true if the entity is allocated.