Implement QCQP duals

[odow]

Closes #295
Closes #309

[codecov]

Codecov Report

Merging #321 into master will increase coverage by 0.73%.
The diff coverage is 95.55%.

@@            Coverage Diff             @@
##           master     #321      +/-   ##
==========================================
+ Coverage   54.08%   54.82%   +0.73%     
==========================================
  Files           7        7              
  Lines        2703     2747      +44     
==========================================
+ Hits         1462     1506      +44     
  Misses       1241     1241              

Impacted Files	Coverage Δ
src/MOI/MOI_wrapper.jl	89.40% <95.55%> (+0.20%)	⬆️
src/gen/libcpx_api.jl	7.39% <0.00%> (+0.11%)	⬆️

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[odow]

Barring objections, I'll leave this open for another 48 hours, then merge.

[metab0t]

I have just noticed that CPLEX.jl is rewritten based on auto-generated bindings with Clang.jl, which makes this package more maintainable. It also makes this PR cleaner. Thanks for your hard work @odow. It's really a new chapter for CPLEX.jl.

[odow]

which makes this package more maintainable. It also makes this PR cleaner

Exactly. It needed to be done. Here's the PR, which contains some discussion you might be interested in reading: #316