Remove union in IndexMap

[blegat]

Follows what the comment says.

[odow]

See the test failures. Is this breaking?

[blegat]

As briefly mentioned in #1245 (comment),
there is an issue with the use of CleverDicts for constraint indices.
By default, CleverDicts uses an anonymous function for inverse_map but that is not an option if you want
to know the concrete type of the CleverDict.
What Gurobi, etc... do is to define a function that maps Int to variable indices and use the type of that function.
For VectorOfConstraints you cannot do that as it depends on the type of the constraints.
Currently, we use Fix1 for that but it does not play well with inference because the constraint type is stored as a DataType:

MathOptInterface.jl/src/Utilities/vector_of_constraints.jl

Line 29 in e0f8b05

Base.Fix1{typeof(CleverDicts.index_to_key),DataType}

This PR provides a non-breaking change that do not create the anonymous function by default and is non-breaking as if the user provides a custom function, the behavior is unchanged.

[odow]

Nice. Does this improve the stress_precompile benchmark?

[blegat]

With JuMP master, with the benchmark of jump-dev/JuMP.jl#2484
Before this PR:

julia> @time Foo.example_diet()
 18.208597 seconds (26.08 M allocations: 1.303 GiB, 4.28% gc time)

After this PR:

julia> @time Foo.example_diet()
 17.757552 seconds (25.81 M allocations: 1.291 GiB, 4.34% gc time)