message: Add a size_hint to MHKeys
  Summary: These hints avoid reallocations in things like map, so super useful to have.
diff: @@ -69,6 +69,14 @@ impl<'a, K: 'a, V: 'a> Iterator for MHKeys<'a, K, V> {
  MHKeys::Map(iter) => iter.next().map(Hashed::key),
  }
  }
  +
  + fn size_hint(&self) -> (usize, Option<usize>) {
  + match self {
  + MHKeys::Empty => (0, Some(0)),
  + MHKeys::Vec(iter) => iter.size_hint(),
  + MHKeys::Map(iter) => iter.size_hint(),
  + }
  + }
  }
  enum MHValues<'a, K: 'a, V: 'a> {
  