feat: add system params for iterating over asset packs (#482)

Closes #272

## Changes

- Add system param `Packs` -- an iterator over asset pack roots
    - Directly addresses #272
- Add system param `AllPacksData` -- a flattened, chained iterator over
listwise data from core and supplementary asset packs
- `AllPacksData::iter_with` takes 2 args: (1) a closure that takes a ref
to the core meta and returns an iterator, (2) a closure takes a ref to a
(supplementary) pack meta and returns an iterator

## Application

`AllPacksData` will be most the most useful of the 2 in Jumpy. There are
a few locations where we iterate over
a list from the core meta and do something, then we iterate over the
packs and for each one iterate over a list. Since `AllPacksData` is a
flattened, chained iterator of all of those sub-iterators, we can turn
this:

```rust
fn maps_sys(asset_server: Res<AssetServer>, game: Root<GameMeta>) {
    for handle in game.core.stable_maps.iter().copied() {
        let map_meta = asset_server.get(handle);
        /* do stuff... */
    }

    for pack in asset_server.packs() {
        let pack_meta = asset_server.get(pack.root.typed::<PackMeta>());
        for handle in pack_meta.maps.iter().copied() {
            let map_meta = asset_server.get(handle);
            /* do stuff... */
        }
    }
}
```

Into this:

```rust
fn maps_sys(asset_server: Res<AssetServer>, all_packs: AllPacksData<GameMeta, PackMeta>) {
    for handle in all_packs.iter_with(
    	|game| game.core.stable_maps.iter().copied(),
    	|pack| pack.maps.iter().copied(),
    ) {
        let map_meta = asset_server.get(handle);
        /* do stuff... */
    }
}
```

demos/asset_packs/Cargo.toml

@@ -0,0 +1,10 @@
+[package]
+name              = "demo_asset_packs"
+edition.workspace = true
+version.workspace = true
+license.workspace = true
+publish           = false
+
+[dependencies]
+bones_framework     = { path = "../../framework_crates/bones_framework" }
+bones_bevy_renderer = { path = "../../framework_crates/bones_bevy_renderer" }

demos/asset_packs/assets/game.yaml

@@ -0,0 +1,4 @@
+# This is our root asset file, which corresponds to our `GameMeta` struct.
+
+title: Asset Packs
+core_items: [1, 2, 3]

demos/asset_packs/assets/pack.yaml

@@ -0,0 +1,4 @@
+# This is the core asset pack file. It's only job is to specify
+# the path to the root asset.
+
+root: ./game.yaml

demos/asset_packs/packs/pack1/data.yaml

@@ -0,0 +1,3 @@
+# This is our root asset file, which corresponds to our `PackMeta` struct.
+
+items: [10, 11, 12]

demos/asset_packs/packs/pack1/pack.yaml

@@ -0,0 +1,9 @@
+# This is the core asset pack file. It's only job is to specify
+# the path to the root asset.
+
+name: Pack 1
+id: pack1_01J9GBGEDY07CT16AQYPKJRKCY
+version: 0.1.0
+game_version: 0.1.0
+root: ./data.yaml
+schemas: []

demos/asset_packs/packs/pack2/data.yaml

@@ -0,0 +1,3 @@
+# This is our root asset file, which corresponds to our `PackMeta` struct.
+
+items: [21, 22, 23]

demos/asset_packs/packs/pack2/pack.yaml

@@ -0,0 +1,9 @@
+# This is the core asset pack file. It's only job is to specify
+# the path to the root asset.
+
+name: Pack 2
+id: pack2_01J9GBGEDYEAX5DMWGNJM9356T
+version: 0.1.0
+game_version: 0.1.0
+root: ./data.yaml
+schemas: []

demos/asset_packs/src/main.rs

@@ -0,0 +1,86 @@
+use bones_bevy_renderer::BonesBevyRenderer;
+use bones_framework::prelude::*;
+
+//
+// NOTE: You must run this example from within the `demos/asset_packs` folder. Also, be sure to
+// look at the `assets/` and `packs/` folders to see the asset files for this example.
+//
+
+/// Our "core" asset type.
+#[derive(HasSchema, Clone, Default)]
+#[repr(C)]
+// We must mark this as a metadata asset, and we set the type to "game".
+//
+// This means that any files with names like `game.yaml`, `game.yml`, `game.json`, `name.game.yaml`,
+// etc. will be loaded as a `GameMeta` asset.
+#[type_data(metadata_asset("game"))]
+struct GameMeta {
+    title: String,
+    core_items: SVec<i32>,
+}
+
+/// Our "supplementary" asset type.
+#[derive(HasSchema, Clone, Default)]
+#[repr(C)]
+#[type_data(metadata_asset("data"))]
+struct PackMeta {
+    items: SVec<i32>,
+}
+
+fn main() {
+    // Setup logging
+    setup_logs!();
+
+    // First create bones game.
+    let mut game = Game::new();
+
+    game
+        // We initialize the asset server.
+        .init_shared_resource::<AssetServer>();
+
+    // We must register all of our asset types before they can be loaded by the asset server. This
+    // may be done by calling schema() on each of our types, to register them with the schema
+    // registry.
+    GameMeta::register_schema();
+    PackMeta::register_schema();
+
+    // Create a new session for the game menu. Each session is it's own bones world with it's own
+    // plugins, systems, and entities.
+    let menu_session = game.sessions.create("menu");
+    menu_session
+        // Install the default bones_framework plugin for this session
+        .install_plugin(DefaultSessionPlugin)
+        // Add our menu system to the update stage
+        .add_system_to_stage(Update, menu_system);
+
+    BonesBevyRenderer::new(game).app().run();
+}
+
+/// System to render the home menu.
+fn menu_system(
+    egui_ctx: Res<EguiCtx>,
+    core_meta: Root<GameMeta>,
+    all_packs: AllPacksData<GameMeta, PackMeta>,
+) {
+    egui::CentralPanel::default()
+        .frame(egui::Frame::none().outer_margin(egui::Margin::same(32.0)))
+        .show(&egui_ctx, |ui| {
+            // Use the title that has been loaded from the asset
+            ui.heading(&core_meta.title);
+
+            ui.separator();
+
+            ui.label(egui::RichText::new("Items from all asset packs:"));
+
+            // Show the numbers from all of the asset packs
+            egui::Grid::new("pack-items").num_columns(1).show(ui, |ui| {
+                for item in all_packs.iter_with(
+                    |core| core.core_items.iter().copied(),
+                    |pack| pack.items.iter().copied(),
+                ) {
+                    ui.label(item.to_string());
+                    ui.end_row();
+                }
+            });
+        });
+}