[Utilities] fix performance of printing functions with many terms

[chriscoey]

_to_string for ScalarAffineFunction and ScalarQuadraticFunction appends each term with s *= ..., so printing a function with n terms is O(n^2). I ran into this printing a model with a large objective:

import MathOptInterface as MOI
model = MOI.Utilities.Model{Float64}()
x = MOI.add_variables(model, 100_000)
f = MOI.ScalarAffineFunction(MOI.ScalarAffineTerm.(1.0, x), 0.0)
MOI.set(model, MOI.ObjectiveSense(), MOI.MIN_SENSE)
MOI.set(model, MOI.ObjectiveFunction{typeof(f)}(), f)
@time sprint(print, model);

On master this allocates 69 GiB. With this PR it allocates about 0.1 GiB, and the printed output is unchanged.