Add Reified set support

[odow]

Part of jump-dev/MathOptInterface.jl#1805 (comment).

Adding to MiniZinc.jl first to work out the kinks before we decide to add to MOI. I think that adding the Reified set with minimal infrastructure might be the way to go. It's a pretty solid/standard set and we already have Indicator, but it'd need a lot of other stuff before it was useful to non-constraint programming solvers.

MiniZinc doesn't support reified constraints for complicated sets like Table, Path, Cumulative, etc. It's been trial and error to see which are supported and which aren't.

• AllDifferent
• BinPacking
• Circuit MiniZinc: evaluation error: Abort: Reified circuit/1 is not supported.
• CountAtLeast
• CountBelongs
• CountDistinct
• CountGreaterThan
• Cumulative
• Path MiniZinc: evaluation error: Abort: Reified path constraint is not supported
• Table
• LessThan
• GreaterThan
• EqualTo

[codecov]

Codecov Report

Merging #13 (2f650d7) into master (de8f1ed) will increase coverage by 1.28%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #13      +/-   ##
==========================================
+ Coverage   93.42%   94.70%   +1.28%     
==========================================
  Files           3        3              
  Lines         289      359      +70     
==========================================
+ Hits          270      340      +70     
  Misses         19       19              

Impacted Files	Coverage Δ
src/MiniZinc.jl	63.63% <100.00%> (+63.63%)	⬆️
src/write.jl	99.26% <100.00%> (+0.22%)	⬆️

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[odow]

Merging this for now, since MOI support might be a while away. If we decide to add Reified to MOI, we can remove from MiniZinc.jl and release a breaking v0.2 release.