message: Box<IndexMap> in SmallMap
  Summary: `SmallMap` is 4 words now instead of 9.
diff: @@ -51,7 +51,7 @@ enum MapHolder<K, V> {
  Vec(VecMap<K, V>),
  // We use a custom hasher since we are only ever hashing a 32bit
  // hash, so can use something faster than the default hasher.
  - Map(IndexMap<Hashed<K>, V, BuildIdHasher>),
  + Map(Box<IndexMap<Hashed<K>, V, BuildIdHasher>>),
  }
  // We define a lot of iterators on top of three other iterators
  @@ -291,7 +291,10 @@ impl<K, V> MapHolder<K, V> {
  fn with_capacity(n: usize) -> Self {
  match VecMap::try_with_capacity(n) {
  Some(vec) => MapHolder::Vec(vec),
  - None => MapHolder::Map(IndexMap::with_capacity_and_hasher(n, Default::default())),
  + None => MapHolder::Map(box IndexMap::with_capacity_and_hasher(
  + n,
  + Default::default(),
  + )),
  }
  }
  @@ -566,7 +569,7 @@ impl<K, V> SmallMap<K, V> {
  where
  K: Eq,
  {
  - let mut holder = MapHolder::Map(IndexMap::with_capacity_and_hasher(
  + let mut holder = MapHolder::Map(box IndexMap::with_capacity_and_hasher(
  capacity,
  Default::default(),
  ));
  