Consider not allocating in the empty archetype on world.spawn()

[alice-i-cecile]

• ex: return something like EntityMutUninit, which turns into EntityMut after an insert or insert_bundle op
• this actually regressed performance last time i tried it, but in theory it should be faster

From Future Work of #1525.

[james7132]

Is this still relevant after #6054? Seems like we're now avoiding this empty Archetype allocation on any non-trivial spawn.