Operator for modifying the world while iterating

Sometimes it's useful to allow components to be inserted/removed and entities to be spawned/despawned in the course of iteration. This is impossible using a conventional interface, but could be achieved with a sufficiently clever interface in the style of Vec::retain. For example:

fn modify<Q: Query, T>(
    &mut self,
    process: impl for<'a> FnMut(<Q::Fetch as Fetch<'a>>::Item) -> T,
    apply: impl FnMut(&mut ModifiableWorld, Entity, T),
)

where the two functions are called, one after the other, for each entity matching Q, and ModifiableWorld is a proxy object that adjusts the iteration state to account for entities being added/removed from the archetype currently being iterated.

The implementation is likely to be a source of significant complexity, and it does not enable anything you can't already do by making two passes and storing intermediate data in a Vec. That said, it would be neat.

The issue names no files or tests; start by tracing the existing query-iteration and world-mutation interfaces, then compare them with the proposed modify signature and Vec::retain approach. Done would require a settled design and an implementation that safely supports inserting, removing, spawning, and despawning during iteration without invalidating traversal.