refactor(3p/nix/libexpr): Use std::string as qualified type

Replaces most uses of `string` with `std::string`.

This came up because I removed the "types.hh" import from
"symbol-table.hh", which percolated through a bunch of files where
`string` was suddenly no longer defined ... *sigh*

third_party/nix/src/libexpr/primops/fetchGit.cc

@@ -215,7 +215,7 @@ static void prim_fetchGit(EvalState& state, const Pos& pos, Value** args,
     state.forceAttrs(*args[0], pos);
 
     for (auto& attr : *args[0]->attrs) {
-      string n(attr.name);
+      std::string n(attr.name);
       if (n == "url")
         url =
             state.coerceToString(*attr.pos, *attr.value, context, false, false);