Macro for spawning related entities can't be used with `SpawnableList`

## What problem does this solve or what need does it fill?

The `children!` macro is convenient, but it can’t be used with `SpawnableList`s. So if I need to use `SpawnWith` or `SpawnIter`, I have to fall back to `Children::spawn`, which isn’t very ergonomic.\

This design affects other crates too. For example, in BEI, the only way to represent presets is by using `SpawnableList` - and those can't be used with the related macro either:

```rust
Bindings::spawn((Spawn(Binding::mouse_motion()), Axial::right_stick()))
```

## What solution would you like?

The perfect solution would be to either adjust the macro or somehow change the design to avoid requiring the use of `Spawn` for `RelationshipTarget::spawn`.