DeepCopy doesn't saturate when there are cycles

[oflatt]

Example test:

#[test]
fn test_deep_copy_terminates() -> Result<(), egglog::Error> {
    let build = "
(let id1 (Id (i64-fresh!)))
(let id-outer (Id (i64-fresh!)))
(let loop 
  (Loop id1
    (UnitExpr id-outer)
    (All (Parallel) (Pair (Boolean id1 false) (Num id1 0)))))
(ExprIsValid loop)
;; The loop is equivalent to zero, so the following holds:
(union (Num id1 0)
       (DeepCopyExpr
        (Loop id1
          (UnitExpr id1)
          (All (Parallel) (Pair (Boolean id1 false) (Num id1 0))))
        (Id (i64-fresh!))))
    ";
    let check = "
    ";
    crate::run_test(build, check)
}

[oflatt]

This test is not valid, because the deep copied loop inputs refer to the outer id instead of id1. We think perhaps inputs referring to ids prevents cycles

[oflatt]

I've now modified the example so that the inputs are valid, and we still create a cycle.

[oflatt]

Closing for now- I prefer we never use deep copy