TLAPM parser silently drops expressions of form `e \in S` when parsing finite set literals

[ahelwer]

Another case of sets being quite difficult to parse in TLA+ in the presence of set map and set filter syntax, similar to tlaplus/tlaplus#1021. The TLAPM syntax parser will parse set literals of the form {e \in S} as the empty set {} - as in, the \in expressions will be silently dropped! Here is some code reproducing the issue:

let _ =
  let open Tlapm_lib in
  let open OUnit2 in
  let (mule : Module.T.mule) =
    "---- MODULE Test ----\nop == {e \\in S}\n===="
    |> module_of_string
    |> Option.get
  in let (set : Expr.T.expr list) =
    match (List.hd mule.core.body).core with
    | Definition ((defn : Expr.T.defn), _, _, _) -> (
      match defn.core with
      | Operator (_, expr) -> (
        match expr.core with
        | SetEnum elements -> elements
        | _ -> failwith "Expected finite set expression"
      )
      | _ -> failwith "Expected operator"
    )
    | _ -> failwith "Expected definition"
  in assert_equal 1 (List.length set) ~printer:Int.to_string

When run using dune runtest, this will produce OUnitTest.OUnit_failure("expected: 1 but got: 0"). This happens no matter how many e \in S expressions you add to the set.

Found during work on #229