Add CanonicalConstraintFunction attribute

[blegat]

For models that knows that the constraint function is already canonical they can just

function MOI.get(model::ModelType, ::Union{MOI.ConstraintFunction, MOI.CanonicalConstraintFunction}, ci)
    ...
end

For MOI.Utilities.Model, the function are not canonicalized when added (should we ?) so we canonicalize them then CanonicalConstraintFunction is called to spare the copy that is done in MOI.Utilities.canonical as it can slows things down dramatically (see jump-dev/SCS.jl#186).
Alternatively, we could say that the functions added to MOI.Utilities.Model are automatically canonicalized in addition of being copied by replacing this copy(f) by canonical(f):

MathOptInterface.jl/src/Utilities/model.jl

Line 550 in 26e0af4

push!(model.constrmap, _add_constraint(model, ci, copy(f), copy(s)))

[odow]

I thought we agreed that solvers should just check if the function was canonicalized. What is the benefit of having ConstraintFunction and CanonicalizedConstraintFunction?

Couldn't we just make it that all solvers return canonicalized functions?

[joaquimg]

Many times we dont need cannonicalized functions.
I think that forcing that all the time is a overkill.
Better to let one use is_cannonical if needed.

[blegat]

Any objection to merge ?

[odow]

Shouldn't the is_canonical check go in this fallback? It would save the checks in Model and UniversalFallback.

[blegat]

Done :)