message: Avoid the boxed iterators in Dict
  Summary: We put in the box because we couldn't figure out how to get rid of it. Now we can, so get rid of it for higher performance (by some miniscule amount).
diff: @@ -105,18 +105,20 @@ where
  self.content.keys().map(|e| e.to_value()).collect()
  }
  - pub fn iter<'a>(&'a self) -> Box<dyn Iterator<Item = (Value<'v>, Value<'v>)> + 'a> {
  - box self
  - .content
  + pub fn iter<'a>(&'a self) -> impl Iterator<Item = (Value<'v>, Value<'v>)> + 'a
  + where
  + 'v: 'a,
  + {
  + self.content
  .iter()
  .map(|(l, r)| (l.to_value(), r.to_value()))
  }
  - pub fn iter_hashed<'a>(
  - &'a self,
  - ) -> Box<dyn Iterator<Item = (Hashed<Value<'v>>, Value<'v>)> + 'a> {
  - box self
  - .content
  + pub fn iter_hashed<'a>(&'a self) -> impl Iterator<Item = (Hashed<Value<'v>>, Value<'v>)> + 'a
  + where
  + 'v: 'a,
  + {
  + self.content
  .iter_hashed()
  .map(|(l, r)| (l.unborrow_copy().to_hashed_value(), r.to_value()))
  }
  