bevy_reflect: Allow #[reflect(skip_serializing)] on enum variant fields

[MrGVSV]

Objective

Fixes #6721

Solution

Converts SerializationData to an enum with variants for structs (including tuple structs) and enums. The latter allows us to store the skipped field info on a per-variant level.

---

Changelog

• Converted SerializationData to an enum
• Added support for #[reflect(skip_serializing)] on enum variant fields

Migration Guide

• SerializationData is now an enum. Custom implementations of this type data will need to be adjusted to account for this:

// OLD
let serialization_data = SerializationData::new([0, 2, 5].into_iter());

// NEW
let serialization_data = SerializationData::Struct(
  StructSerializationData::new([0, 2, 5].into_iter())
);

[nicopap]

LGTM, just too minor readability comments.

[nicopap]

This function signature 😅 . I see what you are doing here, but could it be possible to do the following instead of having a TFields parameter?

        TVariant: Iterator<Item = (TName, &[usize])>,

[MrGVSV]

Hm, I'm having difficulties making it work. It's giving me errors saying it's missing type information.

I'll just leave it as is unless you or someone else has a better way of handling it haha

[bas-ie]

Backlog cleanup: closing due to inactivity, noting existing tracking issue #6721.