Use EntityWorldMut::insert_by_ids

[Shatur]

Currently, we insert components one by one, which causes a lot of archetype moves and behaves somewhat unexpectedly with observers/hooks.
To solve this, we could pass a special buffer for components instead of using Commands, and after all insertions, call EntityWorldMut::insert_by_ids.

[UkoeHB]

If bevy implements command batching, this would no longer be a perf issue (although observers might still run separately).