Move iterable methods from array to iterable traits

[herndlm]

this makes more sense to me, having this in the array traits felt a bit weird already in #1811

Also fixes the new iterable methods via IterableType and ObjectType

[herndlm]

needed a couple more nudges than I thought because of the usage of JustNullableTypeTrait
UPDATE: I think I was misusing JustNullableTypeTrait by using other traits inside it which is a bit weird maybe and could block future types somehow potentially. I moved use statements out of it and rather use NonArrayTypeTrait and NonIterableTypeTrait explicitly which feels "safer". The couple of lines saved was not worth it.

Now I'm happy with this :)

[herndlm]

fyi the failure is unrelated, there seems to be a new baseline update needed for composer on 1.9.x

[ondrejmirtes]

Thank you.