Use cconvert for Optimizer

[odow]

Closes #106

[codecov]

Codecov Report

Merging #108 into master will decrease coverage by 0.06%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##           master     #108      +/-   ##
==========================================
- Coverage   64.14%   64.08%   -0.07%     
==========================================
  Files           3        3              
  Lines         463      465       +2     
==========================================
+ Hits          297      298       +1     
- Misses        166      167       +1     

Impacted Files	Coverage Δ
src/MOI_wrapper/MOI_wrapper.jl	86.66% <91.66%> (-0.26%)	⬇️

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