feat: add system params for iterating over asset packs

[nelson137]

Closes #272

Changes

• Add system param Packs -- an iterator over asset pack roots
  • Directly addresses Create Convenient Way to Iterate Over Casted Asset Packs #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:

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:

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... */
    }
}

[MaxCWhitehead]

Nice, this definitely is an improvement, thanks!