Migrate Quat reflection strategy from "value" to "struct"

[Trashtalk217]

Adopted from #8954, co-authored by @pyrotechnick

Objective

The Bevy ecosystem currently reflects Quat via "value" rather than the more appropriate "struct" strategy. This behaviour is inconsistent to that of similar types, i.e. Vec3. Additionally, employing the "value" strategy causes instances of Quat to be serialised as a sequence [x, y, z, w] rather than structures of shape { x, y, z, w }.

The comments surrounding the applicable code give context and historical reasons for this discrepancy:

// Quat fields are read-only (as of now), and reflection is currently missing
// mechanisms for read-only fields. I doubt those mechanisms would be added,
// so for now quaternions will remain as values. They are represented identically
// to Vec4 and DVec4, so you may use those instead and convert between.

This limitation has since been lifted by the upstream crate, glam.

Solution

Migrating the reflect strategy of Quat from "value" to "struct" via replacing impl_reflect_value with impl_reflect_struct resolves the issue.

Changelog

Migrated Quat reflection strategy to "struct" from "value"
Migration Guide

Changed Quat serialization/deserialization from sequences [x, y, z, w] to structures { x, y, z, w }.

[alice-i-cecile]

@Trashtalk217 CI has a failing test: I think you need to update a serialization test string to match the new impl :)

[Trashtalk217]

Something strange happened where on my end the serialization was returning glam::f32::sse2::quat::Quat instead of glam::Quat, deserialization was very much the same thing. I'm assuming that was just a mistake on my end though.