Idempotency error for module with type constraint

[Niols]

Topiary as of 518c679 hits an idempotency error on

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

because it formats it as:

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

and then as:

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

This is probably just the same problem as #1052.