Remove Union Flattening

[KabirSamsi]

Perhaps this merits further discussion, but having gone through this a few times, I can see no genuine advantage of making the set type recursive. We currently have

type set =
  | Class of clss
  | Union of set list

I propose it to be rewritten as

type set =
  | Class of clss
  | Union of clss list

That is to say, Union([Class 'A'; Union([Class 'B'; Class 'C'])]) should be treated as equivalent to Union([Class 'A'; Class 'B'; Class 'C']). The latter is much easier and cleaner to work with.

Part of my work this semester involved formally proving that dequeue-side semantics for flattened unions behave the same as recursive ones (i.e., showing that the two sets I described above behave the same way). So I'd feel confident in removing that.