Optimization of sum of variables

[MaxOstrowski]

Currently flatzinc seems to support only one variable in the optimization constraint.
If a sum of variables is requested to be minimized an intermediate variable needs to be introduced.
For translational solvers like chuffed it could be beneficial to have a set of variables with coefficients in the optimization statement.
Can this be handled via a translation or directly in flatzinc? Do we need to change the encodings for this? Make an issue in the minizinc github.