Archetypal ECS implementation

[itsBuggingMe]

I'm opening this draft PR for feedback.

Changes

Fixed sparse set implementation, added some collections, and implemented the ECS.
Entity creation, component Add<T>/Remove<T>, Has<T>, Get<T>, TryGet<T>, querying

The current state is pretty agnostic and the System API is unfinished, as I am not sure what kind of API is desired.

Also, would like suggestions on API naming. Currently its whatever popped into my head at the moment.

Notes:

• currently the API is generics only
• no entity tags
• 255 max component count on entity
• Not sure where to put ThrowHelper
• Entity name interferes with terraria's entity
• Probably has bugs (no tests yet)

API sample

World world = new World();

Entity e = world.Entity()
    .Set<int>(69)
    .Set<double>(42)
    .Set<float>(420)
    .Entity;

e.Add<decimal>(4);
e.Remove<decimal>();

_ = e.World;

Query query = world.Query()
    .With<int>()
    .With<double>()
    .Build();

[mayli4]

255 component limit is a non-issue. In a terraria context, there will more than likely never be a need for more than maybe ~20 components per entity.

As for the naming interference, id suggest a global using for either terrarias entity class or our own.

global using Terraria_Entity = Terraria.Entity;
global using EE_Entity = EndlessEscapade.Core.ECS.Entity;

[itsBuggingMe]

Another question I have. Some of the APIs aren't really meant to be messed with by the user; e.g. World.Table. Normally I would simply mark it as internal, but that won't have much of an effect here.
How much of a problem would this be?

[shnakamura]

Looking good apart from minor issues that are mostly syntax/formatting, but obviously easy fixes.