Collecting into a vec leads to better performance for `iter_combinations_mut`

Yes this is definitely faster, especially as the immediate performance boost was apparent even without a release build which was required before!

The updated collision check system is: -

```rust
fn handle_verlet_collisions(
mut query: Query<(&mut Transform, &VerletProperties)>,
) {
let mut entities = query.iter_mut().collect::, &VerletProperties)>>();
for i in 1..entities.len() {
let (a, b) = entities.split_at_mut(i);
let (ref mut tr1, pr1) = a.last_mut().unwrap();
for (ref mut tr2, pr2) in b.iter_mut() {
let mut norm = tr2.translation - tr1.translation;
let dist = norm.length();
if dist < pr1.radius + pr2.radius {
let overlap = (pr1.radius + pr2.radius) - dist;
norm = norm.normalize();
if !pr1.pinned {
tr1.translation.x -= norm.x * overlap * 0.5f32;
tr1.translation.y -= norm.y * overlap * 0.5f32;
}
if !pr2.pinned {
tr2.translation.x += norm.x * overlap * 0.5f32;
tr2.translation.y += norm.y * overlap * 0.5f32;
}
}
}
}
}
```

I wonder if something similar could be done to `iter_combinations_mut` in cases where `K == 2`?

_Originally posted by @polaris64 in https://github.com/bevyengine/bevy/discussions/14685#discussioncomment-10296740_