Remove 'using namespace std' from SerialTypes.h

This unfortunately cuases a ton of flow-down changes because a lot of other
code was depending on that definition existing. This has a fairly large chance
to break builds of external plugins, considering how many internal ones it broke.

testing/btest/plugins/writer-plugin/src/Foo.h

@@ -29,7 +29,7 @@ protected:
 	virtual bool DoHeartbeat(double network_time, double current_time)	{ return true; }
 
 private:
-    string path;
+	std::string path;
 	ODesc desc;
 	threading::formatter::Formatter* formatter;
 };