Implement IntoIterator for Query

[Yokinman]

Whenever I come back to my Bevy projects after a while, something that I always find to be awkward/unintuitive is adding mutable parameters to a query. For example, in the simplest possible case you need to add four mentions of mut:

fn system(items: Query<&Item>) {
    for item in &items {
        // ..
    }
}

fn system(mut items: Query<&mut Item>) {
    for mut item in &mut items {
        // ..
    }
}

• Query<&mut Item> is absolutely fine, no problems.
• mut items is only necessary because you need a mutable reference to items.
• &mut items is only necessary because you can only construct the query's iterator from a mutable reference to it.
• mut item is needed since it's secretly Mut<T> instead of &mut T, but that's mostly fine.

As I see it, two of these would be unnecessary if there was a way to consume the query itself into an iterator. e.g.

fn system(items: Query<&mut Item>) {
    for mut item in items {
        // ..
    }
}

Personally I find this easier to read and remember, and I'd imagine the reduced tedium of the transformation would be better for prototyping ideas or just quickly debugging.