message: Make Symbol non-public
  Summary: Mod `symbol_map` and `SymbolMap` are not public, make `Symbol` explicitly non-public to make it clear.
diff: @@ -66,7 +66,7 @@ fn promote_hash(x: SmallHashResult) -> u64 {
  /// A pre-hashed string used for efficient dictionary lookup.
  #[derive(Clone, Trace)]
  -pub struct Symbol {
  +pub(crate) struct Symbol {
  hash: u64,
  len: usize,
  payload: Box<[u64]>,
  