refs #937 Resolved clazy warnings: rule of three.

src/blackmisc/collection.h

@@ -106,6 +106,9 @@ namespace BlackMisc
         //! Move assignment.
         CCollection &operator =(CCollection && other) noexcept(std::is_nothrow_move_assignable<T>::value) { m_pimpl.reset(other.m_pimpl.take()); return *this; }
 
+        //! Destructor.
+        ~CCollection() = default;
+
         //! Create a new collection with a specific implementation type.
         //! \tparam C Becomes the collection's implementation type.
         //! \param c Initial value for the collection; default is empty, but it could contain elements if desired. The value is copied.