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/iterable/drop_while.cpp

@@ -7,7 +7,7 @@ Distributed under the Boost Software License, Version 1.0.
 #include <boost/hana/detail/assert.hpp>
 #include <boost/hana/detail/constexpr.hpp>
 #include <boost/hana/integral.hpp>
-#include <boost/hana/list/instance.hpp>
+#include <boost/hana/tuple.hpp>
 #include <boost/hana/range.hpp>
 using namespace boost::hana;
 using namespace literals;