WIP: Base.show for Model

[mlubin]

Demo:

using JuMP, GLPK

m = Model(with_optimizer(GLPK.Optimizer))
@variable(m, x[1:2] >= 0)
@constraint(m, c, sum(x) <= 1)
julia> m
A JuMP Model
Variables: 2
`MathOptInterface.SingleVariable`-in-`MathOptInterface.GreaterThan{Float64}`: 2 constraints
`MathOptInterface.ScalarAffineFunction{Float64}`-in-`MathOptInterface.LessThan{Float64}`: 1 constraints
Model mode: Automatic
CachingOptimizer state: EmptyOptimizer
Solver name: SolverName() attribute not implemented by the optimizer.
Names registered in the model: c, x

julia> JuMP.optimize!(m)
julia> m
A JuMP Model
Variables: 2
`MathOptInterface.SingleVariable`-in-`MathOptInterface.GreaterThan{Float64}`: 2 constraints
`MathOptInterface.ScalarAffineFunction{Float64}`-in-`MathOptInterface.LessThan{Float64}`: 1 constraints
Model mode: Automatic
CachingOptimizer state: AttachedOptimizer
Solver name: SolverName() attribute not implemented by the optimizer.
Names registered in the model: c, x

Depends on jump-dev/MathOptInterface.jl#565.

[blegat]

@mlubin Ok to merge this so that I can rebase #1646 on top ?

[mlubin]

It's missing tests but I guess it's ok to merge.

[blegat]

There is a test in #1646 that I can modify to make the output match

[codecov]

Codecov Report

Merging #1602 into master will decrease coverage by 0.41%.
The diff coverage is 20.83%.

@@            Coverage Diff             @@
##           master    #1602      +/-   ##
==========================================
- Coverage   90.93%   90.51%   -0.42%     
==========================================
  Files          28       28              
  Lines        3782     3805      +23     
==========================================
+ Hits         3439     3444       +5     
- Misses        343      361      +18

Impacted Files	Coverage Δ
src/macros.jl	89.89% <ø> (ø)	⬆️
src/objective.jl	93.33% <ø> (ø)	⬆️
src/print.jl	73.07% <0%> (-5.68%)	⬇️
src/JuMP.jl	75% <62.5%> (-1.05%)	⬇️

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