Huge reorganization and refactoring.

- Split type class instances into separate files
- Instances provided automatically by a type class are actually MCDs
- Test each instance in a single file, not one file per method
- Refactor the operator system to fix the ADL-related bug.

example/foldable/unpack.cpp

@@ -5,7 +5,7 @@ Distributed under the Boost Software License, Version 1.0.
  */
 
 #include <boost/hana/functional.hpp>
-#include <boost/hana/list/instance.hpp>
+#include <boost/hana/tuple.hpp>
 
 #include <cassert>
 using namespace boost::hana;