Allow using MiniZincSet from JuMP

[blegat]

Note that I had to create dummy variable equal to a constant in order to fit in MiniZincSet in the packing example. We could add support to specify that some entries are constant to work around it. It can be a follow-up PR though, no need to complicate this one.

I'm planning to use this packing example on my tutorial on Wednesday.
I can use the development version of the package for that part if it's not merged, no rush.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.40%. Comparing base (7484893) to head (69c73a0).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #82      +/-   ##
==========================================
+ Coverage   97.78%   98.40%   +0.61%     
==========================================
  Files           3        3              
  Lines         496      500       +4     
==========================================
+ Hits          485      492       +7     
+ Misses         11        8       -3     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.