Consider unexporting the Systems type

[mewmew]

To minimize the public API to the core concepts of ECS, I propose we unexport the Systems type. It may be considered an internal detail of the ecs package, and should users of ecs want to use a similar priority queue outside of World, the implementation of the sort.Sort interface is trivial.

The Systems type is only used by the private systems field of the World structure, and the Systems method on World returns []System rather than Systems. Therefore we may easily unexport it, without breaking too many users. And, obviously the time to make changes to core packages like ecs is now since #13 was just merged, and all users have to update their code anyways.

From world.go

type World struct {
    systems Systems
}

// Systems returns a list of Systems
func (w *World) Systems() []System {
    return w.systems
}

[EtienneBruines]

SGTM.

[mewmew]

SGTM.

Would you mind me preparing a pull-request?