Move create_unique_class util to the memory namespace

Move create_unique_class util to the memory namespace

create_unique_class is helpful to write unique_ptr wrappers and is now
accessed in numerous places even though it lives in the `details`
namespace.

Moving it to the `memory` namespace to live with other memory management
facilities.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Id4bb5100c1eb3a7e1e2d65f5b2d40ff8f97c786e