[odow]

Closes #3124
Closes #3125

In a fresh session:

julia> import MathOptInterface as MOI

julia> @time begin
           model = MOI.Utilities.Model{Float64}()
           @time MOI.empty!(model)
           x = MOI.add_variable(model)
           @time MOI.delete(model, x)
       end
  0.000012 seconds (49 allocations: 2.250 KiB)
  0.000013 seconds (15 allocations: 736 bytes)
  0.002748 seconds (338 allocations: 17.609 KiB, 89.91% compilation time)

[odow]

Before

julia> import MathOptInterface as MOI

julia> @time begin
           model = MOI.Utilities.Model{Float64}()
           x = MOI.add_variable(model)
           for i in 1:3
               MOI.add_constraint(model, 1.0 * x, MOI.GreaterThan(sin(i)))
           end
           @time MOI.delete(model, x)
       end
 14.950712 seconds (45.77 M allocations: 2.307 GiB, 2.46% gc time, 100.00% compilation time)
 14.959136 seconds (45.77 M allocations: 2.307 GiB, 2.46% gc time, 100.00% compilation time)

julia> @time begin
           model = MOI.Utilities.Model{Float64}()
           x = MOI.add_variable(model)
           for i in 1:3
               MOI.add_constraint(model, 1.0 * x, MOI.GreaterThan(sin(i)))
           end
           @time MOI.delete(model, x)
       end
  0.000006 seconds (19 allocations: 832 bytes)
  0.000119 seconds (134 allocations: 6.594 KiB)

After

julia> import MathOptInterface as MOI

julia> @time begin
           model = MOI.Utilities.Model{Float64}()
           x = MOI.add_variable(model)
           for i in 1:3
               MOI.add_constraint(model, 1.0 * x, MOI.GreaterThan(sin(i)))
           end
           @time MOI.delete(model, x)
       end
  0.020555 seconds (78.89 k allocations: 4.176 MiB, 99.71% compilation time)
  0.028824 seconds (80.25 k allocations: 4.240 MiB, 98.74% compilation time)

julia> @time begin
           model = MOI.Utilities.Model{Float64}()
           x = MOI.add_variable(model)
           for i in 1:3
               MOI.add_constraint(model, 1.0 * x, MOI.GreaterThan(sin(i)))
           end
           @time MOI.delete(model, x)
       end
  0.000015 seconds (25 allocations: 960 bytes)
  0.000130 seconds (140 allocations: 6.719 KiB)

[odow]

@blegat's #3125 is

julia> import MathOptInterface as MOI

julia> @time begin
           model = MOI.Utilities.Model{Float64}()
           x = MOI.add_variable(model)
           for i in 1:3
               MOI.add_constraint(model, 1.0 * x, MOI.GreaterThan(sin(i)))
           end
           @time MOI.delete(model, x)
       end
  0.068279 seconds (154.22 k allocations: 7.852 MiB, 99.85% compilation time)
  0.076540 seconds (155.57 k allocations: 7.916 MiB, 99.48% compilation time)

julia> @time begin
           model = MOI.Utilities.Model{Float64}()
           x = MOI.add_variable(model)
           for i in 1:3
               MOI.add_constraint(model, 1.0 * x, MOI.GreaterThan(sin(i)))
           end
           @time MOI.delete(model, x)
       end
  0.000012 seconds (28 allocations: 992 bytes)
  0.000112 seconds (143 allocations: 6.750 KiB)

So this PR is a clear win over that.