refactor(3p/nix/libexpr): Use absl::node_hash_set in SymbolTable r/794

This replaces the previous use of std::unordered_set with
absl::node_hash_set.

This type was chosen because the current implementation requires
pointer stability.

This does not yet touch the 'Attr' struct.

As a bonus, the implementation of the SymbolTable struct is now
consolidated into a single header/implementation file pair.

0 files changed, 0 insertions, 0 deletions