[Utilities] tidy Base overloads in functions.jl

[odow]

@blegat there's some inconsistent behavior with how we deal with mixed coefficient types. What would you like to happen?

Here's the latest release version (NOT this PR):

(jmp) pkg> st -m MathOptInterface
      Status `/private/tmp/jmp/Manifest.toml`
  [b8f27783] MathOptInterface v1.17.1

julia> x = MOI.VariableIndex(1)
MOI.VariableIndex(1)

julia> 2 * x
(0) + (2) MOI.VariableIndex(1)

julia> (2 * x) + 3
(3) + (2) MOI.VariableIndex(1)

julia> (2 * x) + 3.0
ERROR: MethodError: no method matching +(::MathOptInterface.ScalarAffineFunction{Int64}, ::Float64)
[...]

julia> (2 * x) * 3.0
0.0 + 6.0 MOI.VariableIndex(1)

julia> (2 * x) - 3.0
ERROR: MethodError: no method matching -(::MathOptInterface.ScalarAffineFunction{Int64}, ::Float64)
[...]

julia> (2 * x) / 3.0
ERROR: MethodError: no method matching /(::MathOptInterface.ScalarAffineFunction{Int64}, ::Float64)
[...]

[odow]

Part of the complexity here because of

MathOptInterface.jl/test/Utilities/functions.jl

Lines 30 to 50 in 9e3a1ee

	# Number-like but not subtype of `Number`
	struct NonNumber
	value::Int
	end
	Base.:*(a::NonNumber, b::NonNumber) = NonNumber(a.value * b.value)
	Base.:+(a::NonNumber, b::NonNumber) = NonNumber(a.value + b.value)
	Base.zero(::Type{NonNumber}) = NonNumber(0)
	function Base.isapprox(a::NonNumber, b::NonNumber; kws...)
	return isapprox(a.value, b.value; kws...)
	end
	function test_NonNumber()
	two = NonNumber(2)
	three = NonNumber(3)
	six = NonNumber(6)
	three_x = MOI.Utilities.operate(*, NonNumber, three, x)
	six_x = MOI.Utilities.operate(*, NonNumber, six, x)
	@test six_x ≈ two * three_x
	@test six_x ≈ three_x * two
	return
	end

Do we really want to define * between things that are not numbers? What does that even mean? Do we have any use-cases of a function with non-number coefficients? How do we know an arithmetic exists?

[blegat]

I don't know of any use-case. We had so many issues with upstream expecting things to be number, e.g., in jump-dev/MutableArithmetics.jl#47 that I wanted to stay away from it when possible. However for MOI coefficients it's probably fine to assumes the coefficients are numbers

[odow]

Do we want + and - to also work for non-numbers? I'll leave * as-is for now, but add a comment about removing it in MOI 2.0.

[odow]

Left as-is for now. There hasn't been a need to add more methods for Base.:+ and Base.:-, so let's hold off until there is a reason.