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/nix-instantiate/nix-instantiate.cc

@@ -62,7 +62,7 @@ void processExpr(EvalState& state, const Strings& attrPaths, bool parseOnly,
         Path drvPath = i.queryDrvPath();
 
         /* What output do we want? */
-        string outputName = i.queryOutputName();
+        std::string outputName = i.queryOutputName();
         if (outputName.empty()) {
           throw Error(
               format("derivation '%1%' lacks an 'outputName' attribute ") %