Add SpawnableList implementation for adding multiple entities per iteration

[Ceedrich]

Objective

When working with complex spawn hierarchies, it can sometimes be useful to spawn multiple for each object in a list. I encountered this, when using bevy_enhanced_input. Consider the following scenario:

struct KeyboardConfig {
  up: [Binding; 3],
  down: [Binding; 3],
  left: [Binding; 3],
  right: [Binding; 3],
}

//Without FlatSpawnVec
impl KeyboardConfig {
  fn x_axis(&self) -> impl SpawnableList<BindingOf> {
    SpawnIter(
      Spawn((self.left.0, Negate::all())),
      Spawn(self.right.0),
      Spawn((self.left.1, Negate::all())),
      Spawn(self.right.1),
      Spawn((self.left.2, Negate::all())),
      Spawn(self.right.2),
    ) 
  }
}


// With FlatSpawnVec
impl KeyboardConfig {
  fn x_axis(&self) -> impl SpawnableList<BindingOf> {
    FlatSpawnVec(self.left.into_iter().zip(self.right)
      .map(|(l, r)| (Spawn((l, Negate::all()), Spawn(r))
      .collect()
    ) 
  }
}

If the bindings didn't have a known size, this would not be possible without a FlatSpawnVec.

Solution

This can be solved by using a FlatSpawnVec implementation that spawns multiple entities per iteration. I could also see the vector being changed in favor of an iterator or similar.

Testing

I added a test bevy_ecs::spawn::test::flat_spawn_vec

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