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/symbol-table.hh

@@ -3,11 +3,9 @@
 #include <absl/container/node_hash_set.h>
 #include <absl/strings/string_view.h>
 
-#include "types.hh"
-
 namespace nix {  // TODO(tazjin): ::expr
 
- // TODO(tazjin): Replace with a simpler struct, or get rid of.
+// TODO(tazjin): Replace with a simpler struct, or get rid of.
 class Symbol {
  private:
   const std::string* s;  // pointer into SymbolTable