Only allow AliasNode as a structural reference

[ssalbdivad]

Currently, the rules about where aliases (for cyclic types) can occur are not strict enough, leading to confusing structures where you don't know when you're going to get an alias or a resolved node.

Now that the ID is calculated before the node itself is created, this could be used to ensure we can resolve to that reference ahead of time and only in structural positions, i.e. on a Sequence/Required/Optional/Index node.