`Fiddle::Pointer#to_value` is unsafe

Well, it's pretty self-explanatory. It returns (VALUE)(data->ptr), and ptr is obviously not marked, so can be GCed. This method is either should be removed completely (it'll still e possible to hack around with Fiddle.dlunwrap, but this at least feels hacky) or to be allowed only for pointers marked to be in fact VALUEs (mark return value type in Function constructor), and they of course should gc_mark their ptrs. Btw, there is no way to create a pointer from Ruby object, either.

Inizia leggendo l’implementazione di Fiddle::Pointer#to_value e del costruttore Function, poi esamina come il marking dei puntatori e Fiddle.dlunwrap interagiscono con il garbage collector di Ruby. L’issue presenta alternative di rimozione e restrizione dei tipi; per completarla sono necessari un design di sicurezza deciso, un’implementazione e la copertura del comportamento interessato.