Remove Union Flattening

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

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

I propose it to be rewritten as

```ocaml
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.

Locate the `set` type definition and all uses of `Union`. Trace how recursive and flattened unions are handled, then verify that the flattened representation preserves the stated dequeue-side semantics. Done means the recursive type is removed and the existing behavior remains equivalent.

Written by the indexing model from the issue text.