Unformatted module with type constraint

[Niols]

As of 518c679, Topiary formats:

let foo () =
  ((module struct
      type s
    end): t)

as:

let foo () =
((module struct
  type s
end): t)

I'm not sure what would be the best way to format this, but at least there should be some indentation. Without the type constraint, things look fine, I suppose:

let foo () =
  (module struct
     type s
   end)

gets formatted as:

let foo () = (module struct
  type s
end)