Add range methods to RingMap and RingSet

[cuviper]

impl<K, V, S> RingMap<K, V, S> {
    pub fn range<R>(&self, range: R) -> Iter<'_, K, V>;
    pub fn range_mut<R>(&mut self, range: R) -> IterMut<'_, K, V>;

    pub fn get_range<R>(&self, range: R) -> Option<(&Slice<K, V>, &Slice<K, V>)>;
    pub fn get_range_mut<R>(&mut self, range: R) -> Option<(&mut Slice<K, V>, &mut Slice<K, V>)>;
}

impl<T, S> RingSet<T, S> {
    pub fn range<R>(&self, range: R) -> Iter<'_, T>;
    pub fn get_range<R>(&self, range: R) -> Option<(&Slice<T>, &Slice<T>)>;
}

All of these are bounded where R: RangeBounds<usize> too.

Note that the get_range and get_range_mut methods are similar to indexmap, as well as our existing Slice methods, except here we need to return a pair of slices.

The range and range_mut methods are more like those on VecDeque, where you don't have to worry about the head+tail pair yourself.