What problem does this solve or what need does it fill?

All of our hierarchy management controls happen through Commands, except methods of sorting children for a given entity. These methods should be changed to Commands like the rest of the hierarchy management. This might be required when changing Children to a relation, anyways.

What solution would you like?

Replace the following methods with commands:

• Children::swap
• Children::sort_by
• Children::sort_by_key
• Children::sort_by_cached_key
• Children::sort_unstable_by
• Children::sort_unstable_by_key

Ideally:

impl EntityCommands {
    pub fn sort_children_by(&mut self, compare: impl FnMut(Entity, Entity) -> Ordering) -> &mut Self;
    pub fn sort_children_by_key<K: Ord>(&mut self, compare: impl FnMut(Entity) -> K) -> &mut Self;
    // etc...
}