Indexes in the heap

I believe there is a better way to find indexes in the interpretation. I'm talking specially about the line in interpretMS
```
let eval_prog verbose prog vlist = 
   ....
   let merge heapval =
      let l = Array.find_index ((=) heapval) heap in (*       TODO : how to find index????       *)
```

This operation is done every time we use a constructor and could be costly