Dive into C++14 - [3] - Generic “unique resource“ wrapper

The third tutorial in the “Dive into C 14“ series covers the implementation of a generic “unique resource“ class, with the same ownership semantics as `std::unique_ptr`. --- `std::unique_ptr` is not limited to pointer-like handles - by adapting other kinds of handles to the `NullablePointer` concept, it can still be used, as shown here: