Add `values_mut` to `{BTree, Hash}Map`

I don't understand why there is a [`values()` method](https://doc.rust-lang.org/std/collections/struct.HashMap.html#method.values), but no `values_mut()` method for map-collections. While `keys_mut()` can't work, `values_mut()` shouldn't be a problem, right?

It would be equivalent to `map.iter_mut().map(|(_, v)| v)`.
