[MOI] Allow accessing multiple results

[odow]

Solves JuMP roadmap item "Accessing multiple results from solvers."

Here's what the test looks like at the JuMP level:

using JuMP, Gurobi, Random, Test

function _is_binary(x; atol = 1e-6)
    return isapprox(x, 0; atol = atol) || isapprox(x, 1; atol = atol)
end

N = 30
Random.seed!(1)
item_weights, item_values = rand(N), rand(N)

model = Model(Gurobi.Optimizer)
@variable(model, x[1:N], Bin, start = 0)
@constraint(model, item_weights' * x <= 10)
@objective(model, Max, item_values' * x)
optimize!(model)
RC = result_count(model)
@test RC > 1

for n in [0, RC + 1]
    @test_throws MOI.ResultIndexBoundsError value.(x; result = n)
    @test_throws MOI.ResultIndexBoundsError objective_value(model; result = n)
end

for n = 1:RC
    xn = value.(x; result = n)
    @test all(_is_binary, xn)
    @test isapprox(
        objective_value(model; result = n),
        item_values' * xn,
        atol=1e-6,
    )
end

Tests pass:

[odow]

Oops. Yeah there was a much simpler implementation.