* Initial version of virtual_overrider_self_life_support (enables safely passing unique_ptr to C++).
* Clang 3.6, 3.7 compatibility.
* Adding missing default constructor.
* Restoring test for exception for the case that virtual_overrider_self_life_support is not used.
* Fixing oversight: Adding missing holder().ensure_was_not_disowned().
* Adding unit tests for new `struct smart_holder` member functions.
* Moving virtual_overrider_self_life_support to separate include file, with iwyu cleanup.