Feature Request: IndexSet entry APIs

[zao111222333]

I wonder whether IndexSet::entry is in the roadmap, if yes, I can try to submit PR.
Like the one in hashbrown::HashSet, for IndexSet it will be

impl<T, S> IndexSet<T, S>
where
    T: Hash + Eq,
    S: BuildHasher,
{
    pub fn entry(&mut self, value: T) -> Entry<'_, T, S> { ... }
}
pub enum Entry<'a,T,S> { ... }

[cuviper]

Yes, but I'll want to match the eventual API of the standard library HashSet, rust-lang/rust#60896. I don't mind having a draft PR waiting for that, if you're so inclined, but it could be a while before that will be merged.