Add entity component system functionality

[anissen]

Integrate Composite into Cosy to be able to use entity component functionality directly as part of the language.

This is especially useful for applications with a lot of objects that can change properties interactively. However, it can also be convenient as a sort of in-memory database for a wider variety of applications.

Limitations:

• Composite library is currently linked in. This only works locally.
• Still work-in-progress.

---

Example usage:

struct Position {
    mut x Num
    mut y Num
}
struct Velocity {
    mut x Num
    mut y Num
}

// create entity
spawn(
    Position { x = 200, y = 200 }, 
    Velocity { x = 1.2, y = -3.4 }
)

// query entities
query mut Position p, Velocity v {
    p.x += v.x * delta
    p.y += v.y * delta
}