[Utilities] fix operate(vcat for VectorNonlinearFunction

[odow]

#2553 got complicated to debug because of this 😄

Without this PR, this code:

MathOptInterface.jl/src/Bridges/Constraint/bridges/NormInfinityBridge.jl

Lines 61 to 68 in 927c58b

	scalars = MOI.Utilities.eachscalar(func)
	t, x = scalars[1], scalars[2:end]
	# Create f_new = [-x; x].
	f_new = MOI.Utilities.operate(vcat, T, MOI.Utilities.operate(-, T, x), x)
	# Add +t to each row of x
	for i in 1:(2*(length(scalars)-1))
	MOI.Utilities.operate_output_index!(+, T, i, f_new, t)
	end

modifies the user's input function.

[blegat]

You don't need to copy if it's the output of scalarize, I would rather copy in the two push! above

[odow]

You don't need to copy if it's the output of scalarize

Ah but you do if you hit this method MOI.Utilities.scalarize(f::MOI.VectorNonlinearFunction) = f.rows.

The question of whether scalarize and vectorize are mutable hasn't really come up before, because they always produced unique objects.

[blegat]

😨