BTreeMap doesn't have IndexMut trait

[abbradar]

As we already have get() and Index, and also get_mut() it's strange not to have IndexMut -- it should be pretty straightforward using get_mut():

impl<'a, K, Q, V> IndexMut<&'a Q> for BTreeMap<K, V>
  where K: Ord + Borrow<Q>, Q: Ord + ?Sized {
    fn index_mut(&mut self, index: &Q) -> &mut Self::Output {
      self.get_mut(index).unwrap()
    }
}

[sfackler]

These have been intentionally left off to make sure that there won't be any breakage if/when IndexAssign traits are added.

[Mark-Simulacrum]

Closing in favor of #25034.