Bevy Map Handles

Map Handle<T> to Handle<U>.

All handles pointing to asset a of type T will point to asset b of type U when mapped.

let mapped = match handle.map_weak::<U>() {
    Err(_) => panic!("asset is pending"),
    Ok(mapped) => mapped,
};

Make it strong with Assets<U> if desired:

mapped.make_strong(assets);