Removed query filter

[alice-i-cecile]

RENDERED

Addresses https://github.com/bevyengine/bevy/issues/2148 by proposing a Removed<C> query filter.

This works exactly as you might hope, using reliable change detection semantics and mechanisms. Full credit to @maniwani (Joy) for the core idea of just storing another component with the change tick.

Some notes:

1. We can't fully remove RemovedComponents, since this doesn't work for despawned entities, since they no longer exist to be queried for.
2. The current design is opt-in, using the machinery from #27, to avoid pointless performance overhead.

[alice-i-cecile]

It was actually written with derive(Component) in mind, but yes, this is a little stale. Let me take a look :)

[maniwani]

One more question this RFC should address is, "What happens to LastRemoved<T> if you insert T again?"

Also, I'm thinking there should be a way to rig this internally so that the options are:

• no removal detection
• ticks only
• ticks + data

Also if the strategy stays as described, I think "no removal detection" should be default if you can't auto-detect it from queries, since LastRemoved<T> being a component will fragment archetypes.