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/ref_test.cpp

@@ -6,7 +6,7 @@
 // run-time test for "boost/ref.hpp" header content
 // see 'ref_ct_test.cpp' for compile-time part
 
-#include <boost/ref.hpp>
+#include <boost/core/ref.hpp>
 #include <boost/core/lightweight_test.hpp>
 
 namespace {