Deprecated boost::core::is_same and the associated header.

Moved is_same implementation to detail (both directory and namespace)
to use in the public headers and avoid introducing new dependencies.
The documentation now recommends users to use Boost.TypeTraits or
C++ standard library instead.

Also, removed unnecessary includes and added missing ones in a few
places.

test/allocator_pocca_test.cpp

@@ -6,7 +6,6 @@ Distributed under the Boost Software License, Version 1.0.
 (http://www.boost.org/LICENSE_1_0.txt)
 */
 #include <boost/core/allocator_access.hpp>
-#include <boost/core/is_same.hpp>
 #include <boost/core/lightweight_test_trait.hpp>
 
 template<class T>