`Spawn<B>` now does an unnecssary archetype move / split insert

[cart]

As of #20772, we are now unnecessarily splitting the spawn((bundle, relationship)) into spawn(relationship).insert(bundle), resulting in an unnecessary archetype move and redundant bookkeeping. This needs to be fixed, as Spawn is a high-traffic api.

This points to missing tools in our toolkit!

[james7132]

To help with those who might try to take a stab at implementing this, making MovingPtr<'a, T: Bundle> : Bundle would be the most "straightforward" way to go about it, but this runs up against Bundle's 'static lifetime requirement, which is required for the TypeId mapping. Finding a way around is the crux of this issue.