Newtype hashbrown (#18694)

# Objective

- Fixes #18690
- Closes [#2065](bevyengine/bevy-website#2065)
- Alternative to #18691

The changes to the Hash made in #15801 to the
[BuildHasher](https://doc.rust-lang.org/std/hash/trait.BuildHasher.html)
resulted in serious migration problems and downgraded UX for users of
Bevy's re-exported hashmaps. Once merged, we need to go in and remove
the migration guide added as part of #15801.

## Solution

- Newtype `HashMap` and `HashSet` instead of type aliases
- Added `Deref/Mut` to allow accessing future `hashbrown` methods
without maintenance from Bevy
- Added bidirectional `From` implementations to provide escape hatch for
API incompatibility
- Added inlinable re-exports of all methods directly to Bevy's types.
This ensures `HashMap::new()` works (since the `Deref` implementation
wont cover these kinds of invocations).

## Testing

- CI

---

## Migration Guide

- If you relied on Bevy's `HashMap` and/or `HashSet` types to be
identical to `hashbrown`, consider using `From` and `Into` to convert
between the `hashbrown` and Bevy types as required.
- If you relied on `hashbrown/serde` or `hashbrown/rayon` features, you
may need to enable `bevy_platform_support/serialize` and/or
`bevy_platform_support/rayon` respectively.

---

## Notes

- Did not replicate the Rayon traits, users will need to rely on the
`Deref/Mut` or `From` implementations for those methods.
- Did not re-expose the `unsafe` methods from `hashbrown`. In most cases
users will still have access via `Deref/Mut` anyway.
- I have added `inline` to all methods as they are trivial wrappings of
existing methods.
- I chose to make `HashMap::new` and `HashSet::new` const, which is
different to `hashbrown`. We can do this because we default to a
fixed-state build-hasher. Mild ergonomic win over using
`HashMap::with_hasher(FixedHasher)`.

Author: bushrat011899

crates/bevy_picking/src/hover.rs

@@ -131,9 +131,7 @@ fn build_over_map(
         .filter(|e| !cancelled_pointers.contains(&e.pointer))
     {
         let pointer = entities_under_pointer.pointer;
-        let layer_map = pointer_over_map
-            .entry(pointer)
-            .or_insert_with(BTreeMap::new);
+        let layer_map = pointer_over_map.entry(pointer).or_default();
         for (entity, pick_data) in entities_under_pointer.picks.iter() {
             let layer = entities_under_pointer.order;
             let hits = layer_map.entry(FloatOrd(layer)).or_default();

crates/bevy_platform_support/Cargo.toml

@@ -14,7 +14,10 @@ default = ["std"]
 # Functionality
 
 ## Adds serialization support through `serde`.
-serialize = ["hashbrown/serde"]
+serialize = ["dep:serde", "hashbrown/serde"]
+
+## Adds integration with Rayon.
+rayon = ["dep:rayon", "hashbrown/rayon"]
 
 # Platform Compatibility
 
@@ -28,10 +31,11 @@ std = [
   "portable-atomic-util/std",
   "spin/std",
   "foldhash/std",
+  "serde?/std",
 ]
 
 ## Allows access to the `alloc` crate.
-alloc = ["portable-atomic-util/alloc", "dep:hashbrown"]
+alloc = ["portable-atomic-util/alloc", "dep:hashbrown", "serde?/alloc"]
 
 ## `critical-section` provides the building blocks for synchronization primitives
 ## on all platforms, including `no_std`.
@@ -57,6 +61,8 @@ hashbrown = { version = "0.15.1", features = [
   "equivalent",
   "raw-entry",
 ], optional = true, default-features = false }
+serde = { version = "1", default-features = false, optional = true }
+rayon = { version = "1", default-features = false, optional = true }
 
 [target.'cfg(target_arch = "wasm32")'.dependencies]
 web-time = { version = "1.1", default-features = false, optional = true }