This is a long commit, because it introduces a new abstraction:
a general std::vector<> of std::unique_ptrs<>.
Moreover, it replaces a number of pointers by C++ references,
when the callee does not suppoert null objects.
This simplifies memory management and makes ownership more
explicit. It is a proof-of-concept and a test-bed for
the other core data structrures.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
