Fix performance issue

[odow]

Closes #172 Closes #173

Before

julia> bench();
Optimal - objective value 4.50015e+08
Optimal objective 450015000 - 0 iterations time 0.002
  4.113598 seconds (60.00 k allocations: 2.976 MiB)

After

julia> bench();
Optimal - objective value 4.50015e+08
Optimal objective 450015000 - 0 iterations time 0.002
  0.000598 seconds (4 allocations: 234.547 KiB)

Script

using Cbc
const MOI = Cbc.MOI

function bench(N = 30_000)
    cbc = Cbc.Optimizer()
    model = MOI.Utilities.CachingOptimizer(MOI.Utilities.Model{Float64}(), cbc)
    x = MOI.add_variables(model, N);
    MOI.add_constraints(model, x, MOI.GreaterThan.(1.0:N));
    MOI.set(model, MOI.ObjectiveSense(), MOI.MIN_SENSE);
    f = MOI.ScalarAffineFunction(MOI.ScalarAffineTerm.(1.0, x), 0.0);
    MOI.set(model, MOI.ObjectiveFunction{typeof(f)}(), f);
    MOI.optimize!(model)
    @time MOI.get.(model, MOI.VariablePrimal(), x)
end
bench()

[codecov[bot]]

Codecov Report

Merging #180 (1a263fe) into master (33bcd1b) will increase coverage by 0.67%. The diff coverage is 95.00%.

@@            Coverage Diff             @@
##           master     #180      +/-   ##
==========================================
+ Coverage   53.93%   54.61%   +0.67%     
==========================================
  Files           3        3              
  Lines         534      542       +8     
==========================================
+ Hits          288      296       +8     
  Misses        246      246              

Impacted Files	Coverage Δ
src/MOI_wrapper/MOI_wrapper.jl	80.83% <95.00%> (+0.47%)	:arrow_up:

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