Computation readback does not recognize postconditions that are not lambdas

This snippet works, even if it seems to be introducing an `exists* (x:unit). Pure False`.

```pulse
fn
test_intro (_:unit)
requires emp
ensures emp
{
intro_exists (fun () -> pure False) ();
admit()
}
```

Removing the admit makes it fail as it should.

```pulse
fn
test_intro (_:unit)
requires emp
ensures emp
{
intro_exists (fun () -> pure False) ()
}
```

Puzzlingly, if I try to print the state before the admit, it seems as if `intro_exists` was not applied since it is bound at type `stt_ghost`.

```pulse
fn
test_intro (_:unit)
requires emp
ensures emp
{
intro_exists (fun () -> pure False) ();
show_proof_state;
admit()
}
```
```
- Tactic logged issue:
- Current context:
emp **
emp
- In typing environment:
[_#2 : stt_ghost unit emp_inames (pure l_False) (fun _ -> exists* (_: unit). pure l_False),
uu___85#1 : unit]
```

Start by reproducing the two Pulse test_intro snippets and inspecting computation readback for intro_exists postconditions that are not lambdas; compare the state shown by show_proof_state. Done means the version without admit is rejected as expected and the proof state reflects the applied intro_exists postcondition.

Written by the indexing model from the issue text.