update Makefile and CI

See merge request !2

The simplification is acheieved by removing the stackUR workaround.
That RA was used to enusure that the nodes that were parts of the
stack do not change themselves -- this is crucial for the safety of
pop and iter operations.

Now this is achieved by using duplicable propositions (∃ q, n ↦ᵢ{q} v)
to ensure that the node are still alive/not freed.

install CI and update build system

See merge request !1

Thanks to Robbert