Rework layers (#313)

# Objective

Fixes #322 (among other things).

Improve and simplify the API used for `CollisionLayers` and `SpatialQueryFilter`.

## Solution

Add a `LayerMask` struct that stores a bitmask used for collision layers. The `groups` and `masks` properties have also been renamed to `memberships` and `filters`, because "groups" and "layers" sound like the same thing, and "masks" might be confusing since it's a single bitmask (just like groups/memberships). The new naming is also what Rapier uses.

A `LayerMask` can be created from a `u32`, a type implementing `PhysicsLayer`, or an array of layers. `CollisionLayers::new` now takes `impl Into<LayerMask>`. All of the following work:

```rust
let layers = CollisionLayers::new(0b00010, 0b0111);
let layers = CollisionLayers::new(GameLayer::Player, [GameLayer::Enemy, GameLayer::Ground]);
let layers = CollisionLayers::new(LayerMask(0b0001), LayerMask::ALL);
```

`LayerMask` also allows us to reduce the number of `CollisionLayers` methods. `contains_group`/`contains_mask`, `add_group`/`add_mask` and so on have been removed in favor of simply calling methods like `add` or `remove` on the actual properties.

```rust
layers.memberships.remove(GameLayer::Environment);
layers.filters.add([GameLayer::Environment, GameLayer::Tree]);

// Bitwise ops also work since we're accessing the bitmasks/layermasks directly
layers.memberships |= GameLayer::Player; // or bitmask directly, e.g. 0b0010
```

The methods mutate in place instead of returning a copy, which fixes #322.

`SpatialQueryFilter` also now uses `LayerMask` for its masks, and its constructors have been improved. `new` has been removed for redundancy, `with_mask_from_bits` has been removed, and instead it has the methods `from_mask`, `from_excluded_entities`, `with_mask`, and `with_excluded_entities`.

---

## Changelog

### Added

- `LayerMask` struct
- `SpatialQueryFilter::from_mask`
- `SpatialQueryFilter::from_excluded_entities`

### Changed

#### `CollisionLayers`

- `groups` and `masks` of `CollisionLayers` are now called `memberships` and `filters`
- `CollisionLayers` stores `LayerMask`s for memberships and filters
- Methods that took `impl IntoIterator<Item = impl PhysicsLayer>` now take `impl Into<LayerMask>`
- Change `CollisionLayers::all()` to constant `CollisionLayers::ALL`
- Change `CollisionLayers::none()` to constant `CollisionLayers::NONE`
- Change `CollisionLayers::all_groups()` to constant `CollisionLayers::ALL_MEMBERSHIPS`
- Change `CollisionLayers::all_masks()` to constant `CollisionLayers::ALL_FILTERS`

#### `SpatialQueryFilter`

- `SpatialQueryFilter` stores a `LayerMask` for collision masks
- `SpatialQueryFilter::with_mask` now takes `impl Into<LayerMask>`
- Rename `SpatialQueryFilter::without_entities` to `with_excluded_entities` for consistency

### Removed

- `CollisionLayers::add_group` and `CollisionLayers::add_mask`
- `CollisionLayers::remove_group` and `CollisionLayers::remove_mask`
- `CollisionLayers::contains_group` and `CollisionLayers::contains_mask`
- `CollisionLayers::groups_bits` and `CollisionLayers::masks_bits` (you could get e.g. `layers.groups.0` instead)
- `SpatialQueryFilter::new`
- `SpatialQueryFilter::with_masks_from_bits`

## Migration Guide

- Replace e.g. every `layers.add_group(...)` with `layers.memberships.add(...)` and so on, and similarly for masks/filters
- Change `CollisionLayers::all()` to `CollisionLayers::ALL`
- Change `CollisionLayers::none()` to `CollisionLayers::NONE`
- Change `CollisionLayers::all_groups()` to `CollisionLayers::ALL_MEMBERSHIPS`
- Change `CollisionLayers::all_masks()` to `CollisionLayers::ALL_FILTERS`
- Change `SpatialQueryFilter::new()` to `SpatialQueryFilter::default()` or use the `SpatialQueryFilter::from_mask` or `SpatialQueryFilter::from_excluded_entities` constructors
- Change `SpatialQueryFilter::without_entities` to `with_excluded_entities`
- Change `CollisionLayers::groups_bits()` and `CollisionLayers::masks_bits()` to `layers.memberships.0` and `layers.filters.0`

crates/bevy_xpbd_3d/examples/cast_ray_predicate.rs

@@ -175,7 +175,7 @@ fn raycast(
         direction,
         Scalar::MAX,
         true,
-        SpatialQueryFilter::new(),
+        SpatialQueryFilter::default(),
         &|entity| {
             if let Ok((_, out_of_glass)) = cubes.get(entity) {
                 return !out_of_glass.0; // only look at cubes not out of glass