unsafely fix extraction of list.Nth

Extracted code now uses `List.nth_opt` instead of `nth`.

Also changed to unsafely convert from `Z.t` to ocaml int. This is bad
and doesn't work in general.

drivers/ocaml64.drv

@@ -53,7 +53,7 @@ theory list.Mem
 end
 
 theory list.Nth
-  syntax function nth "List.nth %1 %2"
+  syntax function nth "List.nth_opt %2 (Z.to_int %1)"
 end
 
 theory list.Append