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/json-to-value.hh

@@ -8,6 +8,6 @@ namespace nix {
 
 MakeError(JSONParseError, EvalError)
 
-    void parseJSON(EvalState& state, const string& s, Value& v);
+    void parseJSON(EvalState& state, const std::string& s, Value& v);
 
 }