Implement solve_all for MiniZinc

[zengjian-hu-rai]

Add option num_solutions

• num_solutions will be default to 1, unless it's set explicitly

• Changed primal_solution to primal_solutions to hold 1+ solutions

• Modified MOI.ResultCount, MOI.TerminationStatus.

• If num_solutions > 1 and we've found the given number of solutions, return MOI.SOLUTION_LIMIT, otherwise MOI.OPTIMAL

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.05% :tada:

Comparison is base (8574b83) 97.32% compared to head (4acf817) 97.37%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #43 +/- ## ========================================== + Coverage 97.32% 97.37% +0.05% ========================================== Files 3 3 Lines 448 457 +9 ========================================== + Hits 436 445 +9 Misses 12 12 ``` | [Files Changed](https://app.codecov.io/gh/jump-dev/MiniZinc.jl/pull/43?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=jump-dev) | Coverage Δ | | |---|---|---| | [src/optimize.jl](https://app.codecov.io/gh/jump-dev/MiniZinc.jl/pull/43?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=jump-dev#diff-c3JjL29wdGltaXplLmps) | `92.90% <100.00%> (+0.48%)` | :arrow_up: |

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[zengjian-hu-rai]

Quite a few change suggestions, but all stemming from the high-level comment:

• Let's just have one num_solutions option, store it in the options dictionary, and always add it to the command line args.

Thanks for the great suggestions! Hopefully I've addressed them all.

[zengjian-hu-rai]

Okay, I added a bunch more changes. But this looks good now. Let me know if you're still happy and I'll merge.

Yes it looks great! thanks so much for polishing!

[odow]

thanks so much for polishing!

No problem. Sorry for being somewhat pedantic, but I hope you'll agree it looks a lot tidier now.

[zengjian-hu-rai]

thanks so much for polishing!

No problem. Sorry for being somewhat pedantic, but I hope you'll agree it looks a lot tidier now.

Yes it does, thanks again.