Add BTreeSet entry APIs to match HashSet

[cuviper]

The following methods are added, along with the corresponding Entry implementation.

impl<T, A: Allocator + Clone> BTreeSet<T, A> {
    pub fn get_or_insert(&mut self, value: T) -> &T
    where
        T: Ord,
    {...}
    pub fn get_or_insert_with<Q: ?Sized, F>(&mut self, value: &Q, f: F) -> &T
    where
        T: Borrow<Q> + Ord,
        Q: Ord,
        F: FnOnce(&Q) -> T,
    {...}

    pub fn entry(&mut self, value: T) -> Entry<'_, T, A>
    where
        T: Ord,
    {...}
}

Tracking issue #133549
Closes rust-lang/rfcs#1490

[rustbot]

r? @Mark-Simulacrum

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[Mark-Simulacrum]

@bors r+

Given precedent on HashSet, I'm going to go ahead and approve even without an explicit ACP. This is all unstable anyway.

[bors]

📌 Commit 21b1ab1 has been approved by Mark-Simulacrum

It is now in the queue for this repository.