refactor(3p/nix/libexpr): Use Abseil collection types for parser state

Replaces the previous uses of the (ordered!) std::map and std::set
with absl::flat_hash_{map|set}.

After some careful reading it seems that there is actually no
ordering dependency on these types, and the (drop-in) replacements
perform slightly better.

Overall this is not fixing a bottleneck, just a driveby thing.

Change-Id: Ided695dc75676bd58515aa9382df0be0a09c565e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1220
Reviewed-by: glittershark <grfn@gws.fyi>
Reviewed-by: isomer <isomer@tvl.fyi>
Tested-by: BuildkiteCI

third_party/nix/src/libexpr/symbol-table.hh

@@ -26,6 +26,11 @@ class Symbol {
   bool empty() const { return s->empty(); }
 
   friend std::ostream& operator<<(std::ostream& str, const Symbol& sym);
+
+  template <typename H>
+  friend H AbslHashValue(H h, const Symbol& c) {
+    return H::combine(std::move(h), c.s);
+  }
 };
 
 // SymbolTable is a hash-set based symbol-interning mechanism.