message: Make reserve on SmallMap do something
  Summary: When this code was first written, we did confirm that IndexMap.reserve didn't do much. Since then IndexMap has been rewritten, and it preallocates memory, so call reserve now.
diff: @@ -557,11 +557,7 @@ impl<K, V> SmallMap<K, V> {
  self.upgrade_vec_to_map(want);
  }
  }
  - MapHolder::Map(_) => {
  - // The reserve on IndexMap is useless - just reserves a single
  - // slot so no benefit to reserving in advance
  - // Nothing to do
  - }
  + MapHolder::Map(mp) => mp.reserve(additional),
  }
  }
  