Struct Components using Ref<T>

[Byteron]

This PR changes components to be structs again.
I found a way to keep the foreach(var (a, b, c) in query) syntax even when using structs.
though, now, instead of directly being of the respective types, a, b and c would be of the types Ref<A>, Ref<B> and Ref<C> respectively.
That means .Value is required to access the actual component struct.

var query = world.Query<Position, Velocity>().Build();
foreach(var (pos, vel) in query)
{
    pos.Value.X += vel.Value.X;
    pos.Value.Y += vel.Value.Y;
}

It's not ideal, but a fair trade between ergonomics and performance. Using structs nets us significant performance improvements down the line, due to better cache line optimisations.

[Byteron]

THis is not going to be merged here, instead I will release a separate ECS library that focuses on high performance