Include what is used in pybind11.h (#2499)

* Include what is used

* Separated the C++ standard library headers and the project headers

* Reordering includes

include/pybind11/pybind11.h

@@ -46,6 +46,11 @@
 #include "detail/class.h"
 #include "detail/init.h"
 
+#include <memory>
+#include <vector>
+#include <string>
+#include <utility>
+
 #if defined(__GNUG__) && !defined(__clang__)
 #  include <cxxabi.h>
 #endif