bevy_reflect: Binary formats (#6140)

# Objective

Closes #5934

Currently it is not possible to de/serialize data to non-self-describing formats using reflection.

## Solution

Add support for non-self-describing de/serialization using reflection.

This allows us to use binary formatters, like [`postcard`](https://crates.io/crates/postcard):

```rust
#[derive(Reflect, FromReflect, Debug, PartialEq)]
struct Foo {
  data: String
}

let mut registry = TypeRegistry::new();
registry.register::<Foo>();

let input = Foo {
  data: "Hello world!".to_string()
};

// === Serialize! === //
let serializer = ReflectSerializer::new(&input, &registry);
let bytes: Vec<u8> = postcard::to_allocvec(&serializer).unwrap();

println!("{:?}", bytes); // Output: [129, 217, 61, 98, ...]

// === Deserialize! === //
let deserializer = UntypedReflectDeserializer::new(&registry);

let dynamic_output = deserializer
  .deserialize(&mut postcard::Deserializer::from_bytes(&bytes))
  .unwrap();

let output = <Foo as FromReflect>::from_reflect(dynamic_output.as_ref()).unwrap();

assert_eq!(expected, output); // OK!
```

#### Crates Tested

- ~~[`rmp-serde`](https://crates.io/crates/rmp-serde)~~ Apparently, this _is_ self-describing
- ~~[`bincode` v2.0.0-rc.1](https://crates.io/crates/bincode/2.0.0-rc.1) (using [this PR](bincode-org/bincode#586 This actually works for the latest release (v1.3.3) of [`bincode`](https://crates.io/crates/bincode) as well. You just need to be sure to use fixed-int encoding.
- [`postcard`](https://crates.io/crates/postcard)

## Future Work

Ideally, we would refactor the `serde` module, but I don't think I'll do that in this PR so as to keep the diff relatively small (and to avoid any painful rebases). This should probably be done once this is merged, though.

Some areas we could improve with a refactor:

* Split deserialization logic across multiple files
* Consolidate helper functions/structs
* Make the logic more DRY

---

## Changelog

- Add support for non-self-describing de/serialization using reflection.


Co-authored-by: Gino Valente <[email protected]>

Author: MrGVSV

crates/bevy_reflect/Cargo.toml

@@ -35,6 +35,8 @@ glam = { version = "0.21", features = ["serde"], optional = true }
 
 [dev-dependencies]
 ron = "0.8.0"
+rmp-serde = "1.1"
+bincode = "1.3"
 
 [[example]]
 name = "reflect_docs"

crates/bevy_reflect/src/enums/enum_trait.rs

@@ -136,6 +136,7 @@ pub struct EnumInfo {
     type_name: &'static str,
     type_id: TypeId,
     variants: Box<[VariantInfo]>,
+    variant_names: Box<[&'static str]>,
     variant_indices: HashMap<&'static str, usize>,
     #[cfg(feature = "documentation")]
     docs: Option<&'static str>,
@@ -156,11 +157,14 @@ impl EnumInfo {
             .map(|(index, variant)| (variant.name(), index))
             .collect::<HashMap<_, _>>();
 
+        let variant_names = variants.iter().map(|variant| variant.name()).collect();
+
         Self {
             name,
             type_name: std::any::type_name::<TEnum>(),
             type_id: TypeId::of::<TEnum>(),
             variants: variants.to_vec().into_boxed_slice(),
+            variant_names,
             variant_indices,
             #[cfg(feature = "documentation")]
             docs: None,
@@ -173,6 +177,11 @@ impl EnumInfo {
         Self { docs, ..self }
     }
 
+    /// A slice containing the names of all variants in order.
+    pub fn variant_names(&self) -> &[&'static str] {
+        &self.variant_names
+    }
+
     /// Get a variant with the given name.
     pub fn variant(&self, name: &str) -> Option<&VariantInfo> {
         self.variant_indices

crates/bevy_reflect/src/enums/variants.rs

@@ -89,6 +89,7 @@ impl VariantInfo {
 pub struct StructVariantInfo {
     name: &'static str,
     fields: Box<[NamedField]>,
+    field_names: Box<[&'static str]>,
     field_indices: HashMap<&'static str, usize>,
     #[cfg(feature = "documentation")]
     docs: Option<&'static str>,
@@ -98,9 +99,11 @@ impl StructVariantInfo {
     /// Create a new [`StructVariantInfo`].
     pub fn new(name: &'static str, fields: &[NamedField]) -> Self {
         let field_indices = Self::collect_field_indices(fields);
+        let field_names = fields.iter().map(|field| field.name()).collect();
         Self {
             name,
             fields: fields.to_vec().into_boxed_slice(),
+            field_names,
             field_indices,
             #[cfg(feature = "documentation")]
             docs: None,
@@ -118,6 +121,11 @@ impl StructVariantInfo {
         self.name
     }
 
+    /// A slice containing the names of all fields in order.
+    pub fn field_names(&self) -> &[&'static str] {
+        &self.field_names
+    }
+
     /// Get the field with the given name.
     pub fn field(&self, name: &str) -> Option<&NamedField> {
         self.field_indices