Try moving the decomposition of the MiniZinc `gecode_cumulatives` constraint into MiniZinc

The [`gecode_cumulatives`](https://github.com/Gecode/gecode/blob/develop/gecode/flatzinc/mznlib/fzn_cumulative.mzn#L40) MiniZinc constraint is used for implementing `fzn_cumulative`. The [registry post function](https://github.com/Gecode/gecode/blob/87a464b9d7d67616c1b4c6b98600d38a782d014b/gecode/flatzinc/registry.cpp#L1210) does a lot of inspection of the arguments, posting different variants of the constraint.

This requires first fixing #80 so that the relevant Gecode scheduling constraints are available in MiniZinc.

One potential benefit of this, is that the required `start[i] + duration[i] = end[i]` constraints would be available for common subexpression elimination for MiniZinc.