message: Fix build with rkyv disabled
diff: @@ -15,15 +15,17 @@ use crate::{
  };
  use indexmap::IndexMap;
  use loupe::MemoryUsage;
  -use rkyv::ser::ScratchSpace;
  #[cfg(feature = "enable-rkyv")]
  use rkyv::{
  - de::SharedDeserializeRegistry, ser::Serializer, ser::SharedSerializeRegistry, Archive,
  - Archived, Deserialize as RkyvDeserialize, Fallible, Serialize as RkyvSerialize,
  + de::SharedDeserializeRegistry, ser::ScratchSpace, ser::Serializer,
  + ser::SharedSerializeRegistry, Archive, Archived, Deserialize as RkyvDeserialize, Fallible,
  + Serialize as RkyvSerialize,
  };
  #[cfg(feature = "enable-serde")]
  use serde::{Deserialize, Serialize};
  -use std::collections::{BTreeMap, HashMap};
  +#[cfg(feature = "enable-rkyv")]
  +use std::collections::BTreeMap;
  +use std::collections::HashMap;
  use std::fmt;
  use std::iter::ExactSizeIterator;
  use std::sync::atomic::{AtomicUsize, Ordering::SeqCst};
  