`TreeIndex` get the nearest two keys

Tree datastructures are great for finding entries closest to a key. I'm not familiar with the internal structure of SCC, but would it be possible to add a method for this? Maybe something like: ```rust /// Result of [`TreeIndex::peek_nearest`] and [`TreeIndex::peek_nearest_with`]. pub enum Nearest<T> { /// The [`TreeIndex`] was empty. Empty, // TODO: Depending on the use case, this might need to be Exact(T, T, T). // But then you would also need ExactAndSmaller and ExactAndGreater. /// There was an exact match for the key. Exact(T), /// The key is inbetweeen these keys Nearest(T, T), /// There is only a key smaller than the key. Smaller(T), /// There is only a key greater than the key. Greater(T), } impl<K, V> TreeIndex<K, V> where K: 'static + Clone + Ord, V: 'static + Clone { /// Returns a guarded reference to the nearest values for the specified key without acquiring locks. /// /// The returned reference can survive as long as the associated Guard is alive. pub fn peek_nearest<'g, Q>(&self, key: &Q, guard: &'g Guard) -> Nearest<(&'g K, &'g V)> where Q: Comparable<K> + ?Sized, { todo!() } /// Peeks at the nearest key-value pairs without acquiring locks. /// /// The `reader` is always called, even if the index is empty. pub fn peek_nearest_with<Q, R, F: FnOnce(Nearest<(&K, &V)>) -> R>( &self, key: &Q, reader: F, ) -> R where Q: Comparable<K> + ?Sized { todo!() } } ``` It can also be really useful to get the first and last key of the index.