ifuel 0 breaks unfolding

The below file works if you comment out the `--ifuel 0`. This seems to be another instance of Pulse gratuitously re-typechecking things plus an unclear dependency on ifuel--the regular F* definition checks just fine with the same options (?!).

```fstar
module UnfoldTc
open Pulse.Nolib
#lang-pulse

#push-options "--ifuel 0"

assume val foo (n: nat) : bool & s:Seq.seq bool { Seq.length s == n }

let bar (n: nat) : slprop =
pure (Seq.index (snd (foo (n + 1))) n == false)

fn baz (n: nat)
requires bar n
{
unfold bar
// Expected term of type slprop
// got term pure (Seq.Base.index (snd (foo (n + 1))) n == false)
}
```

Reproduce the `UnfoldTc` example using `#lang-pulse` and `--ifuel 0`, then trace the unfolding and type-checking path for `bar` and `baz`. Done means the shown `baz` function accepts the unfolded `bar` definition with `--ifuel 0`, matching the behavior without that option.

Written by the indexing model from the issue text.