Remove Deref to XYZ from Vec3

[alice-i-cecile]

Bevy users routinely report confusing, frustrating errors with terrible compiler messages when attempting to do basic operations on transforms.

For example, this code fails:

#[derive(Component)]
pub struct Position(Vec3);

pub fn player_controls(
    keyboard_input: Res<Input<KeyCode>>,
    mut query: Query<(&Player, &mut Position)>
) {
    let (player, mut position) = query.single_mut();
    let mut velocity = Vec3::ZERO;
    if keyboard_input.pressed(KeyCode::W) {
        velocity.x += 1.;
    }
    if keyboard_input.pressed(KeyCode::S) {
        velocity.x -= 1.;
    }
    if keyboard_input.pressed(KeyCode::D) {
        velocity.y += 1.;
    }
    if keyboard_input.pressed(KeyCode::A) {
        velocity.y -= 1.;
    }
    *position.0 = *position.0 + velocity.normalize();
}

With the following error message:

error[E0369]: cannot add `bevy::prelude::Vec3` to `bevy::math::XYZ<f32>`
  --> src/player.rs:41:31
   |
41 |     *position.0 = *position.0 + velocity.normalize();
   |                   ----------- ^ -------------------- bevy::prelude::Vec3
   |                   |
   |                   bevy::math::XYZ<f32>

This can be solved by removing the * dereferences from that line.

Tracing the root of this problem, we encounter a Deref and DerefMut implementation of Vec3 to XYZ.

By changing this conversion to be explicit, rather than (mis)using Deref, we can dramatically improve the user experience and error messages.

[alice-i-cecile]

For more context as to why this is hard, see https://github.com/bitshifter/glam-rs/blob/main/ARCHITECTURE.md#inner-types-and-traits and https://github.com/bitshifter/glam-rs/blob/main/ARCHITECTURE.md#component-access-via-deref :(

[bitshifter]

Going to close this as addressing it will require rewriting glam internals which is covered by #177. Note that types backed by SIMD such as Vec3A, Vec4, Quat and Mat2 would still require Deref so it will never completely go away but Vec3 and Vec2 could avoid it if code generation is changed.