style(3p/nix): Remove 'using std::*' from types.hh

It is considered bad form to use things from includes in headers, as
these directives propagate to everywhere else and can make it
confusing.

types.hh (which is includes almost literally everywhere) had some of
these directives, which this commit removes.

third_party/nix/src/libexpr/eval.hh

@@ -180,10 +180,10 @@ class EvalState {
      set with attribute `type = "derivation"'). */
   bool isDerivation(Value& v);
 
-  std::optional<string> tryAttrsToString(const Pos& pos, Value& v,
-                                         PathSet& context,
-                                         bool coerceMore = false,
-                                         bool copyToStore = true);
+  std::optional<std::string> tryAttrsToString(const Pos& pos, Value& v,
+                                              PathSet& context,
+                                              bool coerceMore = false,
+                                              bool copyToStore = true);
 
   /* String coercion.  Converts strings, paths and derivations to a
      string.  If `coerceMore' is set, also converts nulls, integers,
@@ -295,11 +295,11 @@ class EvalState {
 };
 
 /* Return a string representing the type of the value `v'. */
-string showType(const Value& v);
+std::string showType(const Value& v);
 
 /* Decode a context string ‘!<name>!<path>’ into a pair <path,
    name>. */
-std::pair<string, string> decodeContext(const std::string& s);
+std::pair<std::string, std::string> decodeContext(const std::string& s);
 
 /* If `path' refers to a directory, then append "/default.nix". */
 Path resolveExprPath(Path path);