Unfold cannot unfold local slprop definitions

```fstar
module LocalUnfold
#lang-pulse
open Pulse

fn foo (x: ref nat) (vx0: erased nat)
requires pts_to x vx0
ensures exists* (vx: nat). pts_to x vx
{
let my_inv = (fun (k: nat) -> pts_to x k);
fold my_inv vx0; // Cannot unfold my_inv vx0, the head is not an fvar
unfold my_inv;
}
```