[Utilities] fix use of recursion in canonicalize!(::ScalarNonlinearFunction)

[odow]

Alternative to #3008

[blegat]

Why is it an alternative ? Do you preserve aliases ?

[odow]

Your one doesn't do any. This PR fixes the use of recursion. There's an open question about aliases etc.

I think this PR would automatically account for aliases because if it reaches duplicate ScalarAffineFunction that need canonicalisation, then the first will update in place and we won't need to check the second.

[blegat]

Good point, canonical! is alias-friendly. I opened #3008 just to make that branch visible as it was needed in jump-dev/JuMP.jl#4032 but it's indeed not implementing the alias-preservation at all yet. I think this PR is an improvement nevertheless so we can merge it

[odow]

I'll fix the formatting in a separate PR.