BTreeMap doesn't have IndexMut trait

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()
    }
}
```
