Use setindex! internally

[blegat]

These were part of #2232

[odow]

Looks like you need to define setindex! for JuMPExtension.

[codecov]

Codecov Report

Merging #2418 (45ece26) into master (a945fb0) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2418      +/-   ##
==========================================
+ Coverage   92.18%   92.23%   +0.04%     
==========================================
  Files          43       43              
  Lines        4595     4596       +1     
==========================================
+ Hits         4236     4239       +3     
+ Misses        359      357       -2     

Impacted Files	Coverage Δ
src/JuMP.jl	78.49% <100.00%> (+1.19%)	⬆️
src/copy.jl	88.23% <100.00%> (ø)
src/macros.jl	92.13% <100.00%> (ø)
src/variables.jl	98.46% <0.00%> (ø)
src/constraints.jl	93.42% <0.00%> (ø)

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