merge src/simulator into src/core; move src/core to new module layout

src/core/model/string.cc

@@ -0,0 +1,8 @@
+#include "string.h"
+
+namespace ns3 {
+
+ATTRIBUTE_CHECKER_IMPLEMENT_WITH_NAME (String, "std::string");
+ATTRIBUTE_VALUE_IMPLEMENT_WITH_NAME (std::string, String);
+
+} // namespace ns3