Allow tuple destruction in anonymous function about gospel

[shym]

A reasonable implementation would probably use patterns there, as we would want to allow also nested tuples, etc.
But patterns do not parse casts at the moment it seems (and they would be required for function arguments):

val f : int option -> int
(*@ r = f x
    requires match x with
             | Some (y:int) -> y >= 0
             | None -> true *)

(* {gospel_expected|
   [125] gospel: internal error, uncaught exception:
                 File "src/dterm.ml", line 332, characters 18-24: Assertion failed
                 
   |gospel_expected} *)