Proving `int =!= (int -> Dv int)`

[mtzguido]

let _ = assert (int =!= (int -> int))
let _ = assert (int =!= (int -> Dv int))

The second assertion here fails, probably due to SMT encoding for effectful arrows. But it seems like this could be made to work easily, int is not a function (effectul or otherwise).

@andricicezar