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

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.

third_party/nix/src/libexpr/nixexpr.cc

@@ -413,14 +413,4 @@ string ExprLambda::showNamePos() const {
       .str();
 }
 
-/* Symbol table. */
-
-size_t SymbolTable::totalSize() const {
-  size_t n = 0;
-  for (auto& i : symbols) {
-    n += i.size();
-  }
-  return n;
-}
-
 }  // namespace nix