* Fixed compilation error when defining function accepting some forms of std::function.

The compilation error happens only when the functional.h header is
present, and the build is done in debug mode, with NDEBUG being
undefined.  In addition, the std::function must accept an abstract
base class by reference.

The compilation error occurred in cast.h, when trying to construct a
std::tuple<AbstractBase>, rather than a std::tuple<AbstractBase&>.
This was caused by functional.h using std::move rather than
std::forward, changing the signature of the function being used.

This commit contains the fix, along with a test that exhibits the
issue when compiled in debug mode without the fix applied.

* Moved new std::function tests into test_callbacks, added callback_with_movable test.