message: Use SmallMap to store heap references
  Summary: It is smaller than `HashSet`.
diff: use std::cell::Cell;
  use std::cell::RefCell;
  use std::cmp;
  -use std::collections::HashSet;
  use std::fmt;
  use std::fmt::Debug;
  use std::fmt::Display;
  @@ -41,6 +40,7 @@ use either::Either;
  use gazebo::cast;
  use gazebo::prelude::*;
  use once_cell::sync::Lazy;
  +use starlark_map::small_set::SmallSet;
  use crate::collections::maybe_uninit_backport::maybe_uninit_write_slice;
  use crate::collections::maybe_uninit_backport::maybe_uninit_write_slice_cloned;
  @@ -125,7 +125,7 @@ pub struct FrozenHeap {
  /// My memory.
  arena: Arena,
  /// Memory I depend on.
  - refs: RefCell<HashSet<FrozenHeapRef>>,
  + refs: RefCell<SmallSet<FrozenHeapRef>>,
  /// String interner.
  str_interner: RefCell<FrozenStringInterner>,
  }
  @@ -136,7 +136,7 @@ pub struct FrozenHeap {
  #[allow(clippy::non_send_fields_in_send_ty)]
  struct FrozenFrozenHeap {
  arena: Arena,
  - refs: HashSet<FrozenHeapRef>,
  + refs: SmallSet<FrozenHeapRef>,
  }
  // Safe because we never mutate the Arena other than with &mut
  