Use batch spawn in benchmarks

[BD103]

Objective

• The benchmarks for bevy_ecs' iter_simple group use for loops instead of World::spawn_batch.
• There's a TODO comment that says to batch spawn them.

Solution

• Replace the for loops with World::spawn_batch.

[hymm]

What is the improvement in the benches from this?

[mockersf]

it shouldn't change the benches has it's in the setup, but would be nice to confirm

[BD103]

I tried benchmarking spawn_batch. Here what I found.

Before:

iter_simple/bd103       time:   [526.69 µs 533.42 µs 540.87 µs]                              
Found 1 outliers among 100 measurements (1.00%)
  1 (1.00%) high mild

After:

iter_simple/bd103       time:   [346.32 µs 347.52 µs 348.79 µs]                              
                        change: [-36.600% -36.049% -35.473%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 1 outliers among 100 measurements (1.00%)
  1 (1.00%) high mild

Code used to test this

// Before
group.bench_function("bd103", |b| {
    use bevy_ecs::prelude::*;
    use glam::*;

    #[derive(Component, Copy, Clone)]
    struct Transform(Mat4);
    #[derive(Component, Copy, Clone)]
    struct Position(Vec3);
    #[derive(Component, Copy, Clone)]
    struct Rotation(Vec3);
    #[derive(Component, Copy, Clone)]
    struct Velocity(Vec3);

    b.iter(|| {
        let mut world = World::new();

        for _ in 0..10_000 {
            world.spawn((
                Transform(Mat4::from_scale(Vec3::ONE)),
                Position(Vec3::X),
                Rotation(Vec3::X),
                Velocity(Vec3::X),
            ));
        }
    })
});

// After
group.bench_function("bd103", |b| {
    use bevy_ecs::prelude::*;
    use glam::*;

    #[derive(Component, Copy, Clone)]
    struct Transform(Mat4);
    #[derive(Component, Copy, Clone)]
    struct Position(Vec3);
    #[derive(Component, Copy, Clone)]
    struct Rotation(Vec3);
    #[derive(Component, Copy, Clone)]
    struct Velocity(Vec3);

    b.iter(|| {
        let mut world = World::new();

        world.spawn_batch(
            std::iter::repeat((
                Transform(Mat4::from_scale(Vec3::ONE)),
                Position(Vec3::X),
                Rotation(Vec3::X),
                Velocity(Vec3::X),
            ))
            .take(10_000)
        );
    })
});

[hymm]

Nice faster and less noisy